Population shrinking is probably a good thing, but population shrinking too quickly might be all sorts of bad.
It’s hard to see where we really are with so many variables, so many future decisions, but I believe we’ve passed the point of “good shrinking” and are well into “all sorts of instability and disruption”. If replacement rate is 2.1 kids/woman, and South Korea is already like 1.1, that’s a huge difference. As current generations pass, each succeeding one will be half its size. That’s a problem.