(Note: I'm not well-steeped in the longtermism literature, so don't look to me as some philosophical ambassador; I'm only commenting since I hadn't seen any other answers yet.)
I get lost with your argument when you say "the standard deviation as a measure of uncertainty [...] could be so large that the coefficient of variation is very small" What is the significance/meaning of that?
I read your Medium post and I think I otherwise understand the general argument (and even share similar concerns at times). However, my response to the argument you lay out there would mainly be as follows: yes, it technically is possible that a civilizational nuclear reset could lead to good outcomes in the long term, but it's also highly improbable. In the end, we have to weigh what is more plausible, and while there will be a lot of uncertainty, it isn't fair to characterize every situation as purely/symmetrically uncertain, and one of the major goals of longtermism is to seek out these cases where it seems that an intervention is more likely to help than to hurt in the long term.
One of the major examples I've heard longtermists talk about is reducing x-risk. You seem to take issue with this point, but I think the reasoning here is tenuous at best. More specifically, consider the example you give regarding "what if nuclear reset leads to a society that is so "enlightened... that they no longer farm animals for food." Does it seem more plausible that nuclear reset will lead to an enlightened society or a worse society (and enormous suffering in the process)? As part of this, consider all the progress our current society has made in the past ~60 years regarding things like lab-grown meat and veganism—and how much progress in these and other fields would be lost in such a scenario. In this case, it seems far more plausible that preventing a nuclear holocaust will be better for the long term future.
This talk and paper discusses what I think are some of your concerns about growing uncertainty over longer and longer horizons.
This is a very interesting paper and while it covers a lot of ground that I have described in the introduction, the actual cubic growth model used has a number of limitations, perhaps the most significant of which is the assumption that it considers the causal effect of an intervention to diminish over time and converge towards some inevitable state: more precisely it assumes |P(St|A)−P(St|B)|→0 as t→∞, where St is some desirable future state and A and B are some distinct interventions at present.
Please correct me if I am wrong about this.
However, the introduction considers not just interventions fading out in terms of their ability to influence future events but often the sheer unpredictability of them. In fact, much like I did, the idea from chaos theory is cited:
But the model does not consider any of these cases.
In any case, by the author's own analysis ( which is based on a large number of assumptions), there are several scenarios where the outcome is not favorable to the longtermist.
Again, interesting work, but this modeling framework is not very persuasive to begin with (regardless of which way the final results point to).