“No reason to have kids if you can collect Social Security when you get old.”
Not remotely true. I don’t know of anyone who had kids “because government will support me when I’m old”. Nor is that the value of kids.
It's all about the "incentives". There are zero incentives to have children in a developed country today.
In fact, you'll get penalized for doing that.
It's much easier not to have children. The government takes care of you in old age.