China's main interest in propping up North Korea is twofold:
1. They make a handy distraction vis-a-vis the United States. But the calculus is shifting, to where China would prefer to see the United States kindly pick up its shit and depart East Asia, which China increasingly sees as its own sphere. North Korean threats give the US an excuse to stick around in force in South Korea and Japan. It's also provided justification for the JSDF to steadily remilitarize, and as of 2010 Japan announced a change in their defensive policies away from a Cold War focus and instead focusing on protecting Japanese claims outside the Home Islands, like in the Senkaku/Diaoyu Islands. China (government and people both) strongly dislike the idea of Japan remilitarizing. They've never forgotten World War II (and indeed, it's kind of hard to forget when it's such a constant trope in Chinese pop culture).
2. An NK collapse would mean millions of refugees streaming across the border looking for food and shelter (but mostly food). There's already a constant trickle of people slipping past the border guards, but so far they've been a minor nuisance. Chinese popular opinion is mostly sympathetic to their plight, but the official policy of the PRC is forced repatriation -- which usually means death or decades in a labor camp for the "crime" of leaving Best Korea without permission. Popular opinion could shift rapidly though, if that trickle became a flood of desperate Koreans.