The only thing that would completely destroy Seoul and its populace would be a nuke. The damage and death toll would be awful, but even after the worst urban fighting, there'd be people left to take prisoner and businesses to ransack for data.
You aren't going to get concessions for taking Seoul hostage when most of the civilians are dead and everything of value in the city is rubble. North Korea knows that. They wouldn't be dumb enough to launch on invasion predicated on an impossibility.