Or you can keep the smaller apartments and make boarding schools more common. Small kids all live in the same room while the ones old enough to need their own space move out completely to a boarding school. Depending on the specifics, may be a much better solution overall.

So like, the idea of traditional family is "have kids raised by an institution, have them visit you once in a while"?

I actually attended high school in that way in Poland, despite it being very rare here. I was 15 when I started. While I think it was great for me, I would not recommend doing it for younger people - you need some basic emotional maturity and safety.

Even then, it would not solve any problems here, really.

Hm. Indeed that is an idea, but I'm not sure if that doesn't come with other downsides that might be undesirable.

I guess this will work for something like 10-20%? Maybe? (Speaking from a very german cultural background)