If you reject both continuity of identity and prioritarianism, then there isn't much left for an argument to appeal to besides aggregate concerns, which lead to a host of empirical questions you outline.
True, and I probably worded my opening paragraph in an unnecessarily aggressive way, given that premises such as accepting/rejecting continuity aren't really correct or wrong as such. My apologies for that.
If there did exist a choice between two scenarios where the only difference related to your concerns, then I do find it conceivable - though maybe unlikely - that those concerns would tip the balance. But I wouldn't expect such a tight balance to manifest itself in any real-world scenarios. (Of course, one could argue that theoretical ethics shouldn't concern itself too much with worrying about its real world-relevance in the first place. :)
I'd still be curious to hear your opinion about the empirical points I mentioned, though.
I'm not sure what to think about the empirical points.
If there is continuity of personal identity, then we can say that people 'accrue' life, and so there's plausibly diminishing returns. If we dismiss that and talk of experience moments, then a diminishing argument would have to say something like "experience-moments in 'older' lives are not as good as younger ones". Like you, I can't see any particularly good support for this (although I wouldn't be hugely surprised if it was so). However, we can again play the normative uncertainty card to jus...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.