People aren't 'born' in the normal sense - instead they are 'fluctuated' into existence as full-grown adults. Instead of normal 'death', people simply dissolve painlessly after a given amount of time. Nobody is aware that at some point in the future they will 'die', and whenever someone does all currently existing people have their memories instantly modified to remove any trace of them.
This scenario is way, way worse than the real world we live in. It's bad enough that some of my friends and loved ones are dead. I don't want to lose my memories of them too. The social connections people form with others are one of the most important aspects of their lives. If you kill someone and destroy all their connections at the same time you've harmed them far more badly than if you just killed them.
Plus, there's also the practical fact that if you are unaware of when you will "dissolve" it will be impossible for you to plan your life to properly maximize your own utility. What if you had the choice between going to a good movie today, and a great movie next week, and were going to dissolve tomorrow? If you didn't know that you were going to dissolve you'd pick the great movie next week, and would die having had less fun than you otherwise could have had.
I'd prefer option 1 in this scenario, and in any other, because the title of the OP is a misnomer, people can't be replaced. The idea that you are "replacing" someone if you create a new person after they die implies that people are not valuable, they are merely containers for holding what is really valuable (happiness, utility, etc.), and that it does not matter if a container is destroyed as long as you can make a new one to transfer its contents into. I completely disagree with this approach. Utility is valuable because people are valuable, not the other way around. A world with lower utility where less people have died is better than a world of higher utility with more death.
Has anyone here ever addressed the question of why we should prefer
(1) Life Extension: Extend the life of an existing person 100 years
to
(2) Replacement: Create a new person who will live for 100 years?
I've seen some discussion of how the utility of potential people fits into a utilitarian calculus. Eliezer has raised the Repugnant Conclusion, in which 1,000,000 people who each have 1 util is preferable to 1,000 people who each have 100 utils. He rejected it, he said, because he's an average utilitarian.
Fine. But in my thought experiment, average utility remains unchanged. So an average utilitarian should be indifferent between Life Extension and Replacement, right? Or is the harm done by depriving an existing person of life greater in magnitude than the benefit of creating a new life of equivalent utility? If so, why?
Or is the transhumanist indifferent between Life Extension and Replacement, but feels that his efforts towards radical life extension have a much greater expected value than trying to increase the birth rate?
(EDITED to make the thought experiment cleaner. Originally the options were: (1) Life Extension: Extend the life of an existing person for 800 years, and (2) Replacement: Create 10 new people who will each live for 80 years. But that version didn't maintain equal average utility.)
*Optional addendum: Gustaf Arrhenius is a philosopher who has written a lot about this subject; I found him via this comment by utilitymonster. Here's his 2008 paper, "Life Extension versus Replacement," which explores an amendment to utilitarianism that would allow us to prefer Life Extension. Essentially, we begin by comparing potential outcomes according to overall utility, as usual, but we then penalize outcomes if they make any existing people worse off.
So even though the overall utility of Life Extension is the same as Replacement, the latter is worse, because the existing person is worse off than he would have been in Life Extension. By contrast, the potential new person is not worse off in Life Extension, because in that scenario he doesn't exist, and non-existent people can't be harmed. Arrhenius goes through a whole list of problems with this moral theory, however, and by the end of the paper we aren't left with anything workable that would prioritize Life Extension over Replacement.