A conventional war between South Korea and North Korea would be as devastating as a nuclear exchange though. It is estimated that 500,000 rounds of artillery would hit Seoul in the first hour of a conflict, unless South Korea preempted the attack.
I think you are relying too much on the idea that North Korea will stay rational in the face of the South Korean onslaught, or that the US will just stand by and allow the equivalent of a nuclear weapon to go off without retaliating.