China actually depends on the US and South Korea and Japan for a huge amount of manufacturing purchases. While we owe them a lot of money, we're not exactly enemies, but more like two rivals who mutually benefit from each other.
NK threatens that ecosystem, and let's be fair, China would rather just own that territory in order to claim more of the sea.
It's in China's best interest to take out NK. US could never invade because it wouldn't only be bad for retaliation to nearby Allies, but it would be a highly contested point and strengthen tensions in the area.
Therefore, the best move is to convince China to just take it in a business move orchestrated by the US and Chinese governments.
China has once recently condemned nk for these arms races, I don't think it will be long before they do something.
