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.