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?
LW in general seems to favor a very far view. I'm trying to get used to that, and accept it on its own terms. But however useful it may be in itself, a gross mismatch between the farness of views which are taken to be relevant to each other is a problem.
It is widely accepted that spreading population beyond earth (especially in the sense of offloading significant portions of the population) is a development many hundreds of years in the future, right? A lot of extremely difficult challenges have to be overcome to make it feasible. (I for one don't think we'll ever spread much beyond earth; if it were feasible, earlier civilizations would already be here. It's a boring resolution to the Fermi paradox but I think by far the most plausible. But this is in parentheses for a reason).
Extending lifespans dramatically is far more plausible, and something that may happen within decades. If so, we will have to deal with hundreds or thousands of years of dramatically longer lifespans without galactic expansion as a relief of population pressures. It's not a real answer to a serious intermediate-term problem. Among other issues, such a world will set the context within which future developments that would lead to galactic expansion would take place.
The OP's point needs a better answer.
offloading from earth becomes very easy when brains are instantiated on silicon.