I don't think you have any idea how many pieces of artillery they have, nor how many tunnels there are going beneath the DMZ. They are ready to cause as much destruction and chaos as their 1950s era arsenal can inflict.

It would take thousands of cruise missiles to even dent their defenses. You can't use bombers until the anti-aircraft systems are down. It's going to take hours to finish the job, and even then who knows what they've launched in that time. Seoul is an easy target, very hard to miss.