Japan is already facing this scenario, and many Western European countries are there as well, There are few “regular” people left in those countries under 30 years old...
Japan is also facing the nasty problem of being stuck on a mountainous island with damn little arable land and less building space, across a short sea from their mortal enemies that could Zerg Rush them in a week if it weren’t for their version of the English Channel. I don’t envy their social situation even if they don’t turn their babies into sushi like we do.