Because if we erase North Korea, they definitely erase Seoul and Tokyo, killing millions and disrupting the global economy, and possibly erase Los Angeles, New York and Washington as well. That threat is the whole reason they got nukes in the first place.
At this pace, if organic progress on their part, at which point will they match or even exceed US technology advancement?
They don't have electricity yet in large parts of the country, and the economy of the entire country is about half that of Albuquerque New Mexico. I wouldn't worry about us all becoming slaves to North Korea.
Not if the US fires a missile at the most critical/valuable target in NK... armed...with just a note reading...
"Final Warming. The next one WILL be armed."