Already happening. Even in developing countries. Less productive young people in the work force, more unproductive old people depending on social security and whatnot. Everything's gonna be fine until the money runs out. Gigantic economic disaster in the making.
There is no nation without natalism.
> This doesn't bode well for the US if it means its population could soon become similar to that of Japan's
It will. As will every other country if nothing is done to heavily incentivize motherhood and family formation.
When I say this people usually downvote me to oblivion for some reason. Maybe they think I'm making this up. Well, I'm not. They should read what actual japanese government officials have been saying about this. They use the word "extinction".