North Korea is a thorn in China’s side, if there was an easy way to get rid of the problem, the Chinese would do it. Their nightmare is to have millions of NoKo refugees streaming into China.
South Korea is in the same predicament as well, if they somehow beat NoKo, then they will have their own issues with refugees.
Eventually South Korea and China will have to become partners to solve the North Korea problem.
And China sure doesn’t want to rebuild the place. It’s such a basket case it would probably be one of the most expensive reconstructions ever. The infrastructure has been in a shambles for years.