As bad as North Korea might be, nobody would benefit if the regime were to suddenly collapse. South Korea has enough problems of its own and has no interest in integrating 25 million impoverished peasants into their economy or trying to guide their transition into the modern world, and refugees would inevitably show up in every corner of the world and be welcomed about as warmly as refugees from anywhere else. It's not so much China that is propping up North Korea, but rather the collective unwillingness of the rest of the world to step up and try to do right by the North Korean people, we'd rather just sit back and throw shade at the Kims instead.