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 just mean our expected degree of diminishing returns are attenuated by * P(continuity of identity)

I agree there are 'investment costs' in childhood, and if there are only costs in play, then our aggregate maximizer will want to limit them, and extending lifetime is best. I don't think this cost is that massive though between having it once per 80 years or once per 800 or similar. And if diminishing returns apply to age (see above), then it becomes a tradeoff.

Regardless, there are empirical situations where life-extension is strictly win-win: so if we don't have loads of children and so we never approach carrying capacity. I suspect this issue will be at most a near-term thing: our posthuman selves will assumedly tile the universe optimally. There are a host of counterveiling (and counter-counterveiling) concerns in the nearer term. I'm not sure how to unpick them.