NK is a bit like Japan at the end of WW2
The assertion that US weight of arms would just overwhelm a large force of conventional soldiers.
The reality is it didn't work the first time in Korea and it didn't work in Vietnam. The reality is up against a large force of dedicated soldiers and irregulars the US (and other western nations) has and will struggle without devastating cost to them.
Wars like Iraq and Afghanistan haven't been a walk in the park and we were lucky with them that the Iraqi army basically capitulated on day 2. When there are several million military members (even poorly trained) that won't just lay down their arms we are in deep ****.
I'm sure if we really were dedicated to winning at all costs then we/the US would win, but as shown in Vietnam and Iraq the political cost outweighs that in all but the most extreme circumstances, and a war with a country that has no intention/ability to ever threaten your own countries way of life will never be extreme circumstances. As soon as the body bags start coming home the pressure will be well and truly on.
Invading NK would be like Invading Japan in 1945.
The invaders would suffer massive casualties, Many hundreds of thousands.
The defenders losses would likely run to millions (Many of whom would be civilians).
We currently live in a world where a Soldier can be convicted of Murder/Manslaughter for shooting an enemy who was already dying and beyond any practical hope.
Our governments today simply to not have the stomach for the sort of Warfare that invading and defeating NK would represent.
And NK knows it!
NK's position is currently pretty secure from attack. Should they really develop a significant ICBM capability they will be pretty well invulnerable. At this point they will be pretty safe attacking the south.
Nobody in the wider world would wish to risk a Nuclear attack defending SK, not even the USA. SK would be given a token defence, and if it failed, Non-SK forces would come to terms and withdraw.