I hear you and I don't necessarily disagree with you but you forget one thing: they are the same people.Unification?
Why would South Korea want 22 million peasants to cross their border seeking repatriation?
Why would China be seeking a "developed" country, (as a unified Korea would then be) on it's own border. It has none at present.
The two largest countries are India and Russia, but Russia is cushioned by Mongolia and Khazakstan.
...........Just a thought.