Yep. This is the problem I've been zeroing in on as well.

Socialized retirement destroys most of the incentive to sacrifice your youth to grow and raise new people. Why sacrifice your youth and money being a parent when you can leech off the labor of other people's kids in retirement?

I think we'll need to eventually experiment with things that try to give some disproportionate benefit to people who have kids. e.g. everyone gets some minimal socialized retirement and healthcare, but your benefits increase above that baseline in proportion to how many children you raised. Maybe your benefits increase at 3+ kids and decrease with 0-1 kids.