but japan has largely been successful at flattening the curve while Korea is in a serious crisis here. if I have to guess, when there are more older people than young, the society destabilizes.

immigration is going to happen regardless of koreans want it or not and I finally understand why the Korean governments has been pushing soft power for so long, they were planning for this scenario.

I don't think the outcome is going to be as bad as some nationalists make it out to be, for sure most immigrants are going to come from other Asian countries but I do not see it say reaching the dysfunctional state of Western Europe with mostly muslim/african migrants. This is not to say that Southeast Asians won't be pushed to the fringes and experience discrimination that puts them on the same anti-host country culture like in France for example.

Japan has a much higher birthrate at 1.3 (still low worldwide, but way ahead of SK, and not that far from other places), so it's not nearly as bad. More recently, Japan has been encouraging increased immigration to make up for the decline.