They do have a chance
At least at first. We only have 37,000 troops in S. Korea, and they are mostly concentrated along the border. The North has a few thousand long range heavy artillery pieces aimed at this area, and they've had several decades to get it aimed. Should they launch an all out attack they could very well overrun and annihilate the troops we now have over there, although our counter strike would be a Hell on Earth and lead to their destruction.