What actually happened was that the mice discovered 2D women.

Japan is only the beginning.

I'd actually wager that something like that is happening now. Japan's not the only place with lowering rates of sex. North America has been tanking over the last couple decades as well. Lowering fertility rates. Take into account the trends with both sexes these days? Women pushing the "stronk independant" and guys seemingly receeding. We might already be in the stages of that mess.

I've got a little theory about people though. Put too many of them in one space together, and they start to fail. A little bit like mob mentality with riots. Looking at things, I think we function at our best, in low numbers and close knit groups. There's a cutoff point to where adding more numbers goes backwards instead of forwards.

You know how they say two heads are better than one? It's a true statement. Toss in too many extra heads though and it just doesn't hold up.

It's already happening.

In Developed Countries where it's almost guaranteed that your child(ren) will live past 10, the fertility rates are far lower than in a developing country (i.e. India).

Now that's down mostly to women (and men) not prioritising having children by having a career and such, availability of contraceptives and education, and other such things that make life in a developed country in a relative paradise compared to a slum in Delhi.