Thing is, from a rational, realpolitik perspective, North Korea is a huge asset for China that they'd be fools to ever give up. It's unlikely that China actually believes that they can really control a nuclear armed NK, but they don't really have to. Every nuclear test they conduct forces America and our allies to expend more resources shoring up defenses, and also makes us look extremely weak for not being able to put a stop to it. And if our commitment to the region wanes, it allows China to step in with the sales pitch that they can protect Asian countries from threats that America doesn't want to deal with, and pull more states into their sphere and away from ours. The Chinese leadership knows that once Kim and his cronies are gone, THEY become the big bad bully on the block that everyone needs to set their differences aside to deal with, and they don't want that fight yet.