Not just a war, a massive humanitarian crisis. If the NK government were to disappear how many needy people would flood over their border and cause massive resource issues?
It’s a weird situation where basically everyone hates NK (I imagine China would be more happy if that was just more of their normal territory) but everyone is terrified of what happens when the NK State dissolves.
SK wants to reunite but that would have to be done slowly to avoid the same issues China is afraid of (ignoring any ‘diehards’ causing terrorism or just unrest).