China does not want any more trouble. North Korea should be absorbed by the vastly more successful South in the private views of the Chinese government. It would make a lot of sense, but a peace treaty between Korea and China, and another one with Japan does not guarantee stability, but 1,000,000 soldiers burning all the oxygen is disproportionate to the potential for goodwill in the world for a resurgent, united Korea.
It would mean the end of fear. It would be the end of poverty for the population of the North. Gradually, things would improve because the North would be able to produce something to exchange with the rest of the world. At the moment all it produces is fear. China may isolate it, but will it change or will it now start to threaten China also?