The world went to 8 billion people under the various forms of capitalism. However, every country that has adopted any of the forms of socialism ended up suffering from a dearth of births.
There is not a single socialist nation that is thriving, in terms of replacing themselves with children.
The only reason we have an increasing population in the US is due to immigration, illegal and otherwise.
In what way is Japan socialist? In what way is Korea or Singapore socialist? They, like the US, are capitalist welfare states. Japan and Korea have some of the lowest birth rates of any countries in the world.
If your idea of a non-socialist country is one without welfare programs, then there would be no capitalist countries on Earth. Certainly the US wouldn't be considered capitalist by libertarian purists.
I’m not sure the two are related.
What happens when you’re poor you can’t afford to do anything so the only fun thing to do is have sex. And you’re not going to have protected sex either.
Then that huge family of 12 grows up and they have fewer children because they have a bit more money and an easier life and want to enjoy life. So they choose not to have a bunch of kids.
Then those kids grow up and decide they only want one kid at most so they can enjoy their life with all their money.
It’s the cycle of a nation.