Everything that NK has done has been rational - about maintaining power for Kim and his regime. Sure it screws the North Korean people, but that's not the point - they've kept their screws on power. Why would they commit suicide now?
The problem is, how do you dismantle the regime, without the country going into complete chaos? Those people have no clue as to what to do, and most likely, both China and South Korea would be faced with a flood of refugees, which neither wants.