Fareed Zakaria has an excellent column asking what happens after North Korea falls: “And at that point, unless there is careful planning among South Korea, China and the United States, all hell will break loose.”
Realistically, coping with the problem required careful planning and international coordination. Unfortunately, careful planning and international coordination aren’t the kind of thing we’re very likely to see emerge. Realistic talk about unification steps on too many political taboos in South Korea, Americans don’t care enough, and the Chinese government has no real way to be credible in dialogue with the US or Korea.
So when the time comes, it’s going to be a giant mess with terrible advance planning even though the eventual collapse of the DPRK is by no means an unpredictable event.