I might need a better title (It has now been updated), but here goes, anyway:
I've been considering this for a while now. Suppose we reach a point where we can live for centuries, maybe even millenia, then how do we balance? Even assuming we're as efficient as possible, there's a limit for how much resources we can have, meaning an artificial limit at the amount of people that could exist at any given moment even if we explore what we can of the galaxy and use any avaliable resource. There would have to be roughly the same rate of births and deaths in a stable population.
How would this be achieved? Somehow limiting lifespan, or children, assuming it's available to a majority? Or would this lead to a genespliced, technologically augmented and essentially immortal elite that the poor, unaugmented ones would have no chance of measuring up to? I'm sorry if this has already been considered, I'm very uneducated on the topic. If it has, could someone maybe link an analysis of the topic of lifespans and the like?
I decided a while back that the next time there's a LW census, I'm going to suggest adding a question like this: "Ceteris NOT being paribus, if you could make all the people in the world invulnerable to diseases, cellular degeneration and similar aging-related problems by pushing a button, would you push it?" I would be very interested in those results.
Limiting lifespan is a time-buying measure at best, and an ineffective one at that; we already have severely limited lifespans, and that hasn't worked out all that shiny from the sustainability standpoint.
The technological elite scenario seems very likely unless there are other radical advances; having the tech to produce biological immortality isn't the same as being able to provide that service to everyone. As things stand, large swaths of the world still can't even get their hands on simple vaccines, and shipping those across the world is a hell of a lot simpler than genesplicing people. And even if it was as simple as shipping X cans of immortality/enhancement pills to all corners of the world, you'd still have on your hands all the luddites raised to believe that genetic engineering and eternal life are sins against whatever.
I would push this button, and I predict so would almost everyone else here. (So I'm interested in hearing why someone wouldn't.) Reasons: