Why does humanity need to grow in population? That doesn't make sense and is an invalid assumption underlying what you're asking for here.
Maybe we shouldn't have those programs structured as they are now. But we do, and a shock to the system will result in unprecedented economic misery and social instability. If we want to talk about how to deal with demographic change, there's an important long-term conversation to be had about preserving the social safety net. But any solution that wants to avoid a potentially-catastrophic economic crisis must have a serious, realistic answer for this. Not building housing is a fundamentally unserious policy, driven by a preference for the suburban lifestyle and the fear of change more than any realistic concerns about sustainability.
Indebted, underemployed people with just a rented bedroom and dysfunctional social safety nets are, for some odd reason, less eager to procreate.
If you don't have enough grandkids, there won't be enough able-bodied people around to wipe the drool off your generation's collective chin and push your age cohort's wheelchair out to the common room. That's the real cost of pulling up the ladder after yourself. You break the generational compact by denying opportunities to young folks, and they not only won't take care of you in your sunset years, but they won't even exist to do anything for you at any price.