Except the global population isn't growing uncontrollably.

The USA has a birth rate just barely above replenishment (and dropping), Europe has dropped below replenishment rate, etc.

Some pretty graphs here: http://andrewducker.dreamwidth.org/2776214.html

I think the real concern with this is not that we’ll have fewer people in Japan (a good thing) but that the system can’t support contraction.

If you have a workforce in their thirties who are still at entry level positions or thereabouts, and a majority of voters between the age of 50 and 80, you end up with a political class that wins elections with pro-retirement and healthcare policies and a tax-base that has no way to pay for it. This is a recipe for disaster and it’s already happening in Japan. It could easily happen in any aging population.

It's not. Current projections are that global population will peak under 10 billion then start declining. China is going to peak in 2026 then start shrinking. Overpopulation is a solved problem for now. The big problem is that now all those people want to live like Americans.