Even more than the macro factors, the 1950s Chinese state was a revolutionary state bent on the worldwide spread of communism.

In the late 1970s, Deng Xiaopeng killed off the last group of leaders trying to keep that sentiment alive (the Gang of Four) and since then they've been all pragmatism, all the time. Any talk of communism is window-dressing, their regime is ideologically committed to doing what's practical.

So even aside from the cost-benefit, they have no ideological incentive to throw their lot in with the North Korean gov't. As I said upthread, the reason they give them aid right now is because a refugee crisis in Manchuria would be inconvenient. Pragmatism.