I've always had significant concerns regarding longtermism and its fundamental assumptions but the skepticism appears to have grown over time. This is despite or perhaps because of greater engagement with the views on the topic, the critiques and the responses . 

I have tried to carefully examine the claims and justifications of longtermist view together with the responses to the standard objections by its proponents  but nonetheless I am still left with the idea that there are serious issues with it that have not been adequately addressed. Before getting to the details of what I believe to be problematic with longtermism, I admit that it is entirely possible that there have been powerful responses to these objections that I am thus far unaware of. Part of writing this and clarifying the issues as precisely as possible is the hope that someone can easily point to comprehensive responses to them in case they exist.
 

My objections can be broadly divided into two categories : the first deals with the assumptions regarding the consequences of an extinction event and second, our ability to control what happens in the far future. In line with the literature on the subject, the first is related to the concept of cluelessness and the second, tractability of affecting the far future. 

First, let me start with what I believe would be the sequence of reasoning leading to the longtermist perspective:

1. There will likely be a very large number of humans and post-humans coming into existence over the long term future (millions and billions of years from now).
2. The welfare of these humans should matter to us from an ethical standpoint. Specifically, if there are actions we can take now that would improve the welfare of these future individuals or more dramatically their survival, they should enter into our present day ethical calculus.
3. The sheer scale of the future of humanity means that even a small positive change over that future has an enormous total value, dwarfing anything else that may concern us presently.
4. The above is true even if we use a conservative estimate of future human lives and discount the their welfare in comparison to our own. 
5. An extinction risk (x-risk) event is defined as one where either entire humanity gets wiped out or its potential drastically reduced
6. In the specific context of x-risk, any action taken to reduce the probability of such an event happening would, based on the previous assumptions, have a huge impact in terms of overall expected value.
7. As such most individuals, societies and governments would be concerned about extinction risk without any knowledge of longtermism because the consequences seem very obviously bad in the time-horizons that people generally think in terms of. 
8. What distinguishes longtermsim then is the claim that (a) extinction risk is even more important than we may naively assume because of its outsized impact on the far future which is generally not factored in and (b) we should be considering efforts to reduce  x-risk events far out in the future as well.
    

Now I am largely in agreement with points 1-4 and 7.  One of my objections (cluelessness) is related to assumption 6. I addition, I do have concerns about 8(b) but I should mention that I am not entirely sure whether that assumption is even correct? Specifically, insofar as extinction risk is concerned, is the goal of longtermism to reduce the probability of such x-risks in the far future too? As opposed to something near  or mid-term (about 100-1000 years).  I tried to find out if the time-horizons for potential x-risk events have been explicitly discussed in longtermism literature but I didn’t come across anything. In the following I assume that assumption 8(b) is part of the framework of longtermism.  

Also, if I am not mistaken, an x-risk event need not wipe out all of life on earth and the definition pertains to humanity’s survival and continuation as a species. In fact, do we know of any sudden catastrophic events that would eliminate all of life on the planet? 

Cluelessness

Let me first deal with the cluelessness problem. What is absolutely clear is that an extinction event would be terrible in the short and mid-term if we considered only the welfare of humans or post-humans. On the other hand, if we included  the welfare of all sentient species with  reasonable choice of weights for each, then it is quite possible that the net value of a persistent state of the universe is in fact net negative (in the context of the enormous number of farmed land animals and fish and the even greater number of animals in the wild). So an extinction event may lead to the net value going from negative to zero. 

However let’s ignore that for the moment and focus on the human race only.  While on a time horizon of up to a 1000-5000 years or maybe even more, the expected value of the extinction event is quite certainly negative and perhaps so in a large way, this is less clear when we look at horizons that are much longer than that. Why might this be the case? 

Let’s consider both the future after such an extinction event and also the counterfactual future not having such an extinction event. 

Post-extinction civilization

One of the intriguing possibilities is the evolution of post-extinction life forms (we've already recognized that is almost impossible that an extinction event will wipe out every life form as it exists today) towards organisms with human level intelligence. Is there any specific reason for discounting the possibility that arthropods or reptiles evolving over millions of years to something that equals or surpasses the intelligence of humans that were last alive? And over time scales of billions, we could enter the possibility of evolution from basic eukaryotes too. 

In fact, the life forms that post-extinction evolution will bring into existence need not resemble modern day humans or have comparable intelligence. The attribute of relevance for assignment for welfare utility is of course the degree of valenced experience that the species is capable of and in that regard there could be a wide variety of organisms possessing those attributes. 

 Needless to mention, the evolution towards something complex is not a certainty but since so much of longtermism rests on tiny probabilities, shouldn't we be factoring in the probability associated with this too (unless one can argue that it is so infinitesimally small that even in the longtermist calculations, it ends up being negligible). 

If we accept the possibility of advanced sentient life post-extinction, then what is the basis to assume that the net lives of such beings would have overall less value than the humans and post-humans of the counterfactual universe where the extinction did not happen?  More specifically, is there any good reason to assume that the odds are in favor of humans even by a little bit? If so, what exactly is the argument for that?

Again, it is very important to note here that such considerations come into play only when we look far into the future, over millions and billions of years. The future is of course imperative for longtermism calculus because it is predicated on the potential for human and post-human existence over such vast time scales into the future.  In other words, the very attribute that is used to argue in favor of longtermism is also what introduces deep uncertainty about any assumptions that could be made regarding the net value.

Averting an extinction event

Conversely, owing to a longtermist driven intervention, if an extinction event were to be averted at some time t_0 in the future, how do we know that the net value of all human (or comparably advanced life forms) that will come into existence beyond t_0 will even be positive? Again, one has to bear in the mind the sheer magnitude of the timescales and the multitude of trajectories that are possible. An argument that is typically advanced here is based on the assumption that future humans will value similar things that we do or be aligned close to it and hence our altruistic and other-regarding tendencies will prevail. This is not very convincing to me for the simple reason that it sounds extremely speculative amidst tremendous uncertainty. 
 

In their paper on the case for strong longtermism, MacAskill-Greaves recognize the cluelessness as a serious problem but nonetheless maintain that it that decisions taken with long term future in mind is ex-ante rational.  Perhaps I have missed something but I am not even sure they offered much in the way of  justification besides observing that it would be overly pessimistic to assume that we would have no idea of what the future would look like. This is not a very convincing argument as it is neither far-fetched nor pessimistic to think that the future in a hundred million years may look utterly different from anything that we can easily extrapolate our assumptions based on modern day human preferences and values.

Tractability

Our ability to alter future events or trajectories in specific ways by taking actions in the present diminishes with the temporal separation between the future we want to influence and us. It is entirely possible that we can come up with effective strategies to minimize asteroid collisions in the near-term (~100 years) but what happens when we think of an extinction event occurring 5000 years from now?  Now if there is a relatively well-defined causal sequence stretching from now until when the event does occur, we can of course come up with ways to disrupt that chain and nullify the threat. An example is the concern around takeover of the world by highly capable AI agents.  Here we can proactively take steps to minimize the probability of that occurring but it should be mentioned that this involves (a) continuous course of actions as long as AI systems will be around to avoid that undesirable future and not merely a one-off decision and (b) these fall  in alignment with near term priorities as well. However, what about phenomena that don't have such a clear causal linkage to the present? 
 

In this context, I’ve come across references to the paper by Christian Tarsney on epistemic challenges to longtermism that claims to address with the washing-out hypothesis. Despite the introductory sections setting up the problem very well (even acknowledging the results from chaos theory in complex systems), the actual model not only does not deal with any of this but has built into it an assumption that human intervention over the next 1000 years will reduce the probability of an extinction event in that period by some p. In other words, the very issue the paper ostensibly claims to provide new insights on is just assumed right at the beginning of the proposed model!  With that as the starting point, the rest of the paper is about estimating the expected value of the future civilizational expansion which is described by a specific model having several free parameters that are then analyzed for various outcomes. While that type of work is interesting (although extremely speculative), it has no bearing on the specific tractability question of how likely is that an action taken today will have the intended effect several hundred years from now when there is no clear causal chain of events one can trace. 

The line of argument that I often see being deployed to justify long term focus relies on the assumption that on expectation, a very small change in probability towards a desirable direction would lead to extremely large gains. Now,leaving aside philosophical objections to arguments based on tiny probabilities, it would seem that this reasoning would apply more forcefully to prevent an extinction in the near term or more generally over a time horizon where we can make a case for having significant control over outcomes. If we accept that, then the natural extension to mitigate such events in the more distant future is to ensure that the importance of such an extinction event is passed on from one generation to the next rather than take specific steps today to directly avert that. Each generation decides the optimal strategies to avoid extinction over a time-window where there is a feasible degree of control over events. 

No real objections after all?

One thing to observe here is that the objections raised above arise in the context of claims about events and welfare of humans and post-humans in the very long-term future. Indeed by definition, that perspective is baked in the very terminology. 

However, as noted earlier, extinction risk considerations in the near-term are extremely important and there is much less uncertainty about the catastrophic  consequences in the near and mid-term. It is also probably true that the existing efforts to mitigate that are insufficient and not commensurate with the magnitude of the risks posed to humanity's survival by such rare events.  If this is the view one takes, there is nothing in what I have said so far that would challenge it in any way.

It is only when we start making claims and assumptions around the prospects, priorities or experiences of humans millions of years from now that we find that the evidence is lacking and the justifications are vague and unconvincing. 

Again, it is possible that there are more sophisticated arguments that have explored these assumptions and objections more rigorously but I happen to be unaware of them.