Kurzgesagt covered this really well:

https://m.youtube.com/watch?v=Ufmu1WD2TSk

At what point (in the population decline) might housing start to become affordable for younger people?

At what point might younger people start leaving South Korea en mass, because they could not cope with the country's ever-growing ratio of retirees to workers?

At what point might the (North) Korean People's Army just move in and take over?