>There's a strong reason to suspect that if we were effectively immortal, our birth rates would drop to sustainable rates, or less.
Really? Seems to me people like children. Maybe not everyone, and maybe not *today*, but at some point in their lives, once they've got the survival thing comfortably under control they're likely to want to have some - biological imperative versus rational opportunity. And if people are immortal then the sustainable birth rate is zero. Occasionally people will die by violence or accident, but that number will be vanishingly small compared to the population size. Which means there must be a universal ban on reproduction except under very specific circumstances. If the average woman is allowed to have only 0.0000000001 children per year you can't allow women them to make that choice for themselves, the birth rate will be far, far too high.