Look, North Korea is much, much more than communism. Nobody cares about the idea that they're a communist country, that's just western rhetoric. The problem is mere geopolitical positioning - China defends NK because it is a buffer zone. By far the easiest border to invade China from is the Korean border. They don't like that SK is buddy-buddy w/ us. Were we to take over NK, they would attack us no matter what, because they'd consider that encroachment, nukes or no nukes. Why the hell do you think they crossed over the border during the Korean War in the first place?
At the same time, they don't like the regime in NK. The problem is that NK is starving because 80% of the land there is mountain and unsuitable for agriculture. To compensate for this, NK is not just importing food from China, but they're terracing too. The problem with THAT, is that since those mountains are being terraced, valuable mineral deposits are going to waste. NK is a mining haven waiting to be exploited, but the government there doesn't want to exploit it. They want to be self-sufficient. They want to survive as their own country.
So the example in the OP was rather poorly chosen, because the stalemate in Korea has almost nothing to do with nuclear arms, but rather the issue of border control.
On the other hand, there is one conflict which will be significantly affected by this defense grid, and that is the feud between India and Pakistan. Both are intent on nuking the other, and the first one that will be protected by this grid will want to launch its nukes at the other country ASAP. That would be rather bad for global stability, so we're more likely to see diplomatic mediation involving the two countries and their benefactors, the US and China respectively (and slightly off-topic, if you ever get the chance to look into it, the relationship between China and Pakistan is absolutely fascinating and one of the most underrated political alliances of the modern era in terms of importance).
In short, a system that renders the nuclear deterrent obsolete would be rather useless, because we've already rendered the nuclear deterrent obsolete through diplomacy. I don't think we'll be launching nukes at each other even if we could.
