Right... shells age. They blow up in the barrel, things like that. Maybe they even intentionally blow up in the barrel. Not that I would suggest sabotage. There's no way South Korean intelligence could possibly infiltrate North Korea ;)

But even so, if there was a serious threat of war, which is unlikely because China would stop North Korea, the US would place assets in the region and as we got close to a confrontation the US and South Korea (and as things are looking, probably Japan) would begin an aerial and missile bombardment to destroy in place North Korean offensive capabilities. Some would get through of course, perhaps thousands or tens of thousands of South Korean casualties, but in the context of a conventional war North Korea's capabilities would be quickly overwhelmed, at least in my opinion.

But honestly, the current status quo works pretty well for everyone except the people of North Korea, but there's not much we can do. It's a tragedy and the blame for that falls squarely on the Soviet Union and Chinese Communist Party.