Perhaps. But that's predicated on the assumption that a new assault would be staged under the same sorts of circumstances that held in 1950 -- and that's not true for a variety of reasons, not least of which is that the South Koreans are prepared for an invasion. Not only have they got the local defenses set up, but they also undoubtedly have troops and equipment positioned in such a manner that they can attack the North Korean flank should they actually invade with a massive assault.
Plus which, it would be very difficult for the North Koreans to gather the requisite forces in secret -- surprise is probably not in the cards for them; the best they could hope for, would be unpreparedness.
Beyond that, is the fact that NK has no hope of gaining or maintaining air superiority, which would leave their large masses of troops open to annihilation from the air.
Also, I have to think that the use of nuclear weapons would pretty much exclude the possibility of a massive assault, simply because the radioactivity in the blast zones would render their invasion routes unusable.
As to Seoul ... it might well be taken, but remember that the South has been planning and preparing for that. For NK to go beyond Seoul would leave them with long flanks, and the North Koreans would have an expensive time trying to protect them.
“Plus which, it would be very difficult for the North Koreans to gather the requisite forces in secret — surprise is probably not in the cards for them”
70% of their active duty Army (est 950K x .7 or about 665K troops) are permantently garrisoned within 90 miles of the DMZ . . . If Kim wanted to get crazy and throw himself headlong into a wall, we wouldn’t have all that much lead time.