So there is a universe out there where the US would have supported/allied with Nazi Germany had it been convenient?

That doesn't have to be the result of it. A more humane regime in NK doesn't mean reunification has to happen. And, part of the reason those US forces are in South Korea is the threat of the North. By threatening US involvement in case of an attack.

In the initial era of the split between North and South Korea, South Korea both was run by a bunch of people who had a history of outright killing leftists, and the United States was involved in similar actions.

The lack of serious offramps to reunification, along with not as huge a delta in quality of life between north and south for a long time (aid from other countries sure helps!), allowed the DPRK to establish itself as its own nation.

Now there is the surveillance state apparatus allowing the DPRK to exist in its current form in perpetuity. And even if tomorrow they showed up and said "let's unify Korea", South Korea (even ignoring all the ideological reasons it might not want to) would likely be unwilling to absorb an extremely poor country and pay for it (see the painful experience of Germany's unification).

There is probably no off ramp that exists unless people are willing to let the elite walk away clean from the situation in one way or another, and it seems hard to imagine such a future.

And if you are a north korean elite and you are allowed to travel to northern china, you will see a place where things are running more smoothly, but you're still going to see places with massive amounts of internal controls and restrictions. So who's offering the upside to some regime change here?

Nearly every authoritarian country starts with people promising good things. A lot also start with rebels fighting against a group that led a massacre. They're underdog groups with popular support.

Then those underdogs take over. They become paranoid about the possibility of being killed themselves, so they repeat the massacres they fought against. A lot of people who supported the new regime think it's just a few remaining enemies being taken out. It won't happen to them. Then the government starts laying out methods to solidify their control. The list of things seen as traitorous and against national interests grows. It becomes a frog in a boiling pot situation. By the time people realize they might be a target, the system is too complicated and widespread to take down alone, and a new generation of youths have been raised knowing only the current system. And to those youths, things are stable. The most terrifying thing to people raised in stability is the idea of losing that stability. So keeping your head down and following the law is much better than absolutely anything else.

And with the absolute control of information that NK has, a significant portion of people really don't even know a better world exists out there. And they're terrified of anyone that even talks about shaking things up.

For the first couple decades while it was ahead of South Korea economically (in large part due to support from China/USSR) it was not that bad but during that time the system of absolute control by the Kim family was setup and once it was up it is too late to really do anything due to how absolute/brutal the control is (you say anything wrong and you and your whole extended family end up in a prison/death camp)

Basically people are willing to put up with a lot if their lives are getting better (economic growth). Problem with that is what kind of system of control an authoritarian government can setup in that period of growth.

Less of brain control, and more like slaughter of anyone who disagrees or rolls their eyes. Read accounts of those who escaped

Empiricism in the face of a totalitarian regime is difficult.

It's funny to say that because we're living in a bubble too.

If both counties sustain their current trajectories, in 50 years it will be NK re-populating and re-developing SK. And the "if" here is mainly about NK, chances of SK getting out of the death spiral are very thin.

Genuine question that I'm trying to learn about - the industrialisation of Japan and South Korea led to huge wealth creation and increases in quality of living. I know some of that is stagnating now and especially in South Korea things are difficult, but why isn't North Korea ever spoken of in those terms rather than always the GDP hit to South Korea?