Why do you expect the population to hold constant? That unsupported assumption is what you base everything on.

Furthermore, if the population stays the same but ages, there will be major ramifications to SS.

Furthermore, if the population remains constant but fewer and fewer people have children, then those who do have children bear more and more of the burden of providing for everybody else’s retirement. Responding with “so what, SS will cover me whether I have children or not” is kind of missing the point. And leads straight back to the first point. In a world that requires people to have a substantial number of children to survive (like our world with SS), economically disadvantaging people who prioritize having children is a huge risk.

The US keeps makes up for its reproduction rate with immigration. It ends up being roughly constant.

Nobody is economically disadvantaging people with kids. The question at hand is how much we as a society are choosing to advantage them with tax benefits and social services. Including free education! Having kids is pretty great.