In other words, I don't think there's a fact of the matter about "if people should die after 100 years, a thousand years, or longer or at all". The question assumes that there's some single answer that works for everyone. That seems unlikely.

Not necessarily true. The question posits the existence of an optimal outcome. It just neglects to mention what, exactly, said outcome would be optimal to. It would probably be necessary to determine the criteria a system that accounts for immortality has to meet to satisfy us before we start coming up with solutions.

The obvious answer is "Everyone dies if and when they feel like it. If you want to die after 100 years, by all means; if you feel like living for a thousand years, that's fine too; totally up to you."

A limited distribution of resources somewhat complicates the issue, and even with nanotechnology and fusion power there would still be the problem of organizing a system that isn't inherently self-destructive.

I think I agree with the spirit of your answer. "We can't possibly figure out how to do that and in any case doing so wouldn't feel right, so we'll let the people involved sort it out amongst themselves.," but there are a lot of problems that can arise from that. There would probably need to be some sort of system of checks and balances, but then that would probably deteriorate over time and has the potential to turn the whole thing upside down in itself. I doubt you'll ever be able to really design a system for all humanity.

And the idea that it's OK to impose a fixed lifespan on someone who doesn't want it is abhorrent.

To you, perhaps. Well, and me. You're intuitions on the matter are not universal, however. Far from it, as our friends's comments show.

My main problems (read: ones that don't rest entirely on feelings of moral sacredness) with such an idea would be the dangerous vulnerability of the system it describes to power grabs, its capacity to threaten my ambitions, and the fact that, if implemented, it would lead to a world that's all around boring (I mean, if you can fix the life spans then you already know the ending. The person dies. Why not just save yourself the trouble and leave them dead to begin with?)