Earlier quoted context omitted.
If China decided they would no longer back North Korea, the regime could be replaced in hours. Most of the time, when toppling regimes, you have to be worried about someone even worse taking over. In this case, I don't see how it could get any worse.
What I imagine would happen is the North Koreans who have suddenly stopped becoming prisoners in their own land trying to enter China, and then China would have to post guards at the border, and be responsible for the mess, including violence, trying to stop them coming in. So the Chinese government probably has an interest in keeping an NK government alive, that way they can claim "He's the evil dictator oppressing…
Many good and normal and moral people would want to self organize and self govern and build something better from the ashes of a fallen country, the problem is that those good and honest people will be manipulated, tricked, and murdered by those who are more brutal and cunning than them.