Actually, the health of our social programs isn't that based on continued population growth; they'd work pretty much the same with a stable population size.

However, the bane of those social programs is quickly increasing lifespans. No matter if population growth is zero or huge, if you're currently below 45, you anyway have to "prepare for yourself" rather than expect them to survive in the current form - unless the retirement age moves beyond 80 within this decade.

I don't see how you can separate the two. The current system used to work because the population was growing and people didn't live as long. Either change would be enough to result in lower benefits.