This. China wants a buffer. And the South has given up on the idea of re-unification as watching Germany lose a decade+ of economic growth after the Wall fell.
So no one wants to start a war/invade, but the everyone wants the Fat Kid to sit down and shut the hell up.
I don't know that the South has a choice in the matter. The Nork government falls, they scream "help us!" open their borders. And for the sake of this argument, say that China sits on their hands for various reasons more important to them than keeping Nork as a buffer state, so the South and U.S. move in to secure "critical military assets" and humanitarian aid...
The South effectively has reunified with the North. (shrug)
It's lose/lose, if the South insists Nork stay an independent nation, but with normalized relations and eventually a normal border, Nork floods the south with economic refugees which will cost the South about as much as reunification does, without the benefits of the territory, or any control or say-so as to what goes on up there.
So unless China gobbles up a fallen Nork, or there's some kind of multi-lateral U.S. China, ROK, and U.N. "special administrative region"-thingy that leaves the Nork's in limbo for decades... I'm not sure the South can keep a failed and now open Nork at arms length.