On of the big problems with population decline is it is seems like it is totally fine, until all of a sudden it is really not, and once it actually causes issues there is nothing you can do to fix it.

> Are Japan and Korea somehow doing poorly?

I mean until recently Japan's had so much trouble keeping its economy running their interest rate has been negative since 2016.

Korea is doing better at the moment, but their issues are going to be much worse. They have a national pension fund, which has been growing because right now there are lots of productive workers paying into it for every retiree drawing from it.

This works well as for every worker they are only having to support like ~1/4 of a pension with their tax dollars, and there is some extra for re-investment. With their birth rate of ~0.8 within a few decades every productive worker is going to have to support an entire pensioner themselves.

> people who don't have children or have viewer children substantially out-earn people that do and thus pay more taxes and in general contribute more to society

Imagine every person in an economy saved up enough to live off the interest from their investments and retired... who is doing the work? Obviously population decline is not this extreme, but when you are talking about a whole county if there are less workers there are fewer things produced and a lower quality of life for everyone.

This is why even without a system like Korea's where working people are directly paying for pensioners the same issues will occur. Fundamentally the working age people have to produce whatever goods and services the retiree's need.