However many elements of the philosophical case for longtermism are independent of contingent facts about what is going to happen with AI in the coming decades.

Agree, though there are arguments from one to the other! In particular:

1. As I understand it, longtermism requires it to be tractable to, in expectation, affect the long-term future ("ltf").[1]
2. Some people might think that the only or most tractable way of affecting the ltf is to reduce extinction[2] risk in the coming decades or century (as you might think we can have no idea about the expected effects of basically anything else on the ltf because effects other than "causes ltf to exist or not" are too complicated to predict).
3. If extinction risk is high, especially from a single source in the near future, it's plausibly easier to reduce. (this seems questionable but far from crazy)
4. So thinking extinction risk is high especially from a single source in the near future might reasonably increase someone's belief in longtermism.
5. Thinking AI risk is high in the near future is a way of thinking extinction risk is high from a ~single source in the near future
6. So thinking AI risk is high in the near future is a reason to believe longtermism.

[1] basically because you can't have reasons to do things that are impossible.

[1] since "existential risk" on the toby ord definition by definition is anything that reduces humanity's potential (&therefore affects the ltf in expectation) I think it'd be confusing to use that term in this context so I'm going to talk about extinction even though people think there are non-extinction existential catastrophe scenarios from AI as well.

I'm a neartermist with 0.01<P(doom from AI)<0.05 on a 30-year horizon. I don't consider myself a doomer, but I think this qualifies as taking AI risk seriously (or at least not dismissing it entirely).

I think of my neartermism as a result of 3 questions:

1. how much x-risk is there from AI?  
   As I said above, I think there's between a 1% and 5% chance of extinction from AI in the next 30 years. In my mind, this is high. If I were a longtermist, this would be sufficient to motivate to me to work on AI safety.
    
2. how bad is x-risk?
   I am sympathetic to person-affecting views, which to me means thinking of x-risk as primarily impacting people (& animals) alive today. I'm also sympathetic to the idea that it's somewhat good to create a positive life. However, I'd really rather not create negative lives, and I think there is uncertainty on the sign of all not-yet-existent lives. As an example of this uncertainty, consider that many people raised in excellent conditions (loving family, great education, good healthcare, good friends) still struggle with depression. Because of this uncertainty and risk-aversion, the non-person-affecting views part of me is roughly neutral on creating lives as an altruistic act.
    
3. how much can I lower x-risk?
   I have a technical skillset and could directly do AI safety work. However, I think most technical AI safety work still accelerates AI and therefore may accelerate extinction. As an example, I believe (weakly! convince me otherwise please!) that RLHF and instruction-tuning led to the current LLM gold rush and that if LLMs were more toxic (aka less safe?) there would be less investment in them right now. Along these lines, I'm not sure that any technical AI safety work done thus far has decreased AI x-risk.
   I think the best mechanism to lowering AI x-risk is to slow down AI development, both to give us more time in the current safe-ish technological world and perhaps time to shift into a paradigm where we can develop clearly beneficial technical safety tools. I imagine this deceleration to primarily happen through policy. Policy is outside my skillset, but I'd happily write a letter to my congressperson.
   If I could lower AI x-risk by 0.0001%  (I think of this as lowering P(doom) from 0.020000 to 0.019999, or 1 part in 20,000), I'd consider this worth 8 billion people * 1e-6 probability = 8e3 = 8000 deaths averted. I think I have better options to add this many QALYs over the course of my life - without the downside risk of potentially accelerating extinction!

Other reasons I'm not a longtermist / I don't do technical AI safety work:

• I aspire to serve the poor and to serve animals rather than neglecting them or being served by them. I'm interested in working on problems that disproportionately impact the poor (eg pandemics) and not problems that would primarily impact the rich or even impact everyone equally (eg AI) in order to provide a preferential option for the poor. I'd like a world where more people live to 60 rather than one where some people live forever.
• I'm risk-averse with my life's work. If I spent my life working on something that seemed like it might be good and ended up being totally useless, I'd consider that a wasted life.
• I'm not impressed by things like the 80K problem profiles page putting "space governance" above "factory farming" and "easily preventable or treatable illness", or the Wytham Abbey purchase, or FTX, or the trend of spending money on elite students in rich countries without evidence rather than on people in poor countries with great evidence of the good that could be done. This is not the sort of altruism I want to be associated with.

I know some people in this category, mostly because they are extremely uncertain over what the best work is on AI risk.

I feel like I am a neartermist mostly because of my studies and my comparative advantages, neartermist seems more likely to be higher in empathetic leaning person (not sure how to phrase this). However, my tech and interaction with applied AI and geoscience has also allow me to recognise the danger for longtermist risks which with let me approach the research and discussion with open-mindedness despite my comparative advantage in neartermist causes. One of the main attractiveness of EA to me originally was because the movement address both of my concerns.