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 his citizens, the mess over there is not our problem!".

I wonder why that's my first thought, rather than the citizens wondering "Well, the government is gone, let's build a collective to self-govern." -- I guess people would rather join a structured society somewhere else rather than try to build one themselves (probably out of nothing, in NK). Not to mention yeah, even if the current asshole is gone, the next asshole is probably busy setting up shop to resume the oppression.