I didn’t say that Social Security wasn’t a factor, I said that it’s not the sole or even most important factor. Women who want an education and careers aren’t going to give it all up and start having kids in their teens even if you did away with Social Security, and the welfare state, nor do people need to breed their own farm hands these days, welfare state or no welfare state.
Go to USA, Japan, South Korea or even Iran, UAE, Qatar.
They are all below replacement rate.
Different societal norms, religions, etc.
But they all have welfare states.