Nobody seems to question the theory that China would fear North Koreans from fleeing into China in the event of regime collapse or whatever.  Why not?  Surely China has, with its one child policy, reduced its population substantially, and it's not as if the North Koreans are that culturally or racially different. The real reason is the same  as when the Chines sent hordes across the Yalu just as the Marines arrived close to their border. China wants a buffer state, and the last time it seemed about to collapse, they sent in their massive  army, Russian T-34s and MIGs. General Douglas MacArthur wanted permission to bomb China.  Truman opted for a limited war,  so the Marines had to walk back south, and the war went on. And on.

700 Canadians held back 5,000 Chinese at  the battle of Kapyong. We suffered 1,558 casualties, 516 dead. No Canadian Chinese died.

China wants a buffer state between South Korea, and is angry with South Korea over its  new U.S-made anti-missile defences.