The Chinese will whisper to Seoul that they will keep NK reined in if SK gets rid of those pesky Americans. Beijing’s end game is to push the US out of the Western Pacific. First SK then Japan will see reason, and kowtow to Beijing. We need to make it clear to Seoul that if they want to point NK at us to spare themselves, then we of course will feel free to defend our own interests no matter what happens to SK.
I think your reasoning is sound.