Both these countries have really high population densities. Japan's is around 330/km2 and South Korea's is about 530/km2.

Just like the UK, they would probably be better off with less people, geopolitical considerations aside.

They're free to choose the path they prefer.

That "peak prosperity" thing is actually capitalism gone awry in my opinion. I'd include India too here - a common pattern that can be seen in all these three Asian countries is the unhealthy work-life balance. Couple that with the world-wide trend that two incomes are now necessary to raise kids in many of these "fast" growing or economically "prosperous" countries, most people are just choosing to have only 1 (or at most 2 kids), and there are some who are also opting not to have any kids. In India, the opposition leader has also lamented that we have already lost advantage of having a younger population because of poor economic planning and policies (by 2030, India will have the world’s largest youth population). Trump won, in large part, because many Americans are now struggling to feel secure with the wages that they earn - they can't afford to buy a house, which many feel is required to start a family. A course correction is required in the world economy, as, while capitalism-consumerism does seem to provide prosperity, it also seems to be consuming societies that adopts it.