I'm no fan of the DPRK regime, but isn't it naive to assume that any state, even a completely mismanaged despotic one, will "inevitably" collapse?
True, they rely on China just for bare survival, but they have their stuff together enough to create a functioning nuclear weapons program in the face of crippling sanctions, so it's not infeasible that they could find a way to continue on if they were totally isolated - maybe they'd kidnap some Monsanto scientists to provide them with GMO and fertilizer technology. Basically, they don't currently bother feeding their people because they don't currently have to, but there's nothing proving they can't adapt if circumstances change.
And I'm not saying they're likely to conquer the world, but with the nuclear deterrent, it's not terribly likely that the US or anyone will invade them without direct cause.