Well, if it turns out to be such a big problem, then we can just kill everyone at a specific age.

That's sarcasm, of course, but IMHO saying that we shouldn't prolong life because of possible consequence X is morally exactly equal to saying that we should kill old people to avoid that consequence X.

For every X, either X really is so bad that it's worth killing people for it; or X shouldn't be considered as a valid argument against longevity.

I didn't say we shouldn't prolong life, I'm saying we should be proactive about the unintended consequences of longer lives in aggregate.

Could you give an example of how to be proactive as such? I agree that being prepared is good, but I don't personally know enough about old-age care / geriatrics / Social Security to really be in favor of or against any particular policy changes.