When I began looking the information up for my post, I started with the idea that putting down scatterable mines and sub-munitions over the whole of the area that the artillery would come from would be doable. It may well be. There is no question that we would be able to bring a great deal of firepower to bear. The problem with relying wholly on the submunitions is that anything sheltered or unexposed will survive, and be capable of then firing. The MRLSs that the North Koreans have can move out, fire their dozen or two dozen rockets, and move back under cover for reloading within three minutes. To take these out requires singular munitions addressed against them.
The counter side of this is that many of N Koreas hardened facilities are 20km or so behind the DMZ, and further, the mountains limit the types of guns and rockets that can actually strike Seoul.
In a general strike, one has the potential of facing 8,000 large caliber artillery rounds hitting the northern part of a city of 10,000,000, in a predesignated fire pattern, in the first minute. One has a lot of loose ends to sweep up very rapidly. This really isn't a military attack, but rather a terror one against civilians.