This is a really interesting analysis; thanks for sharing. I asked a general question, and you gave a general answer, so I'm not trying to pick holes in your statement. That said, I think the picture you paint is more negative than is necessitated when one considers all the evidence.
I think North Korea is a special case because of its relationship to China, which precludes a stronger response. While many would-be dictators will be looking at Kim Jong-un as a blueprint for how to succeed at the "asshole" game, few of them can expect the the protection of the worlds (soon-to-be) greatest superpower.
If we analyse the situation from the Chinese point of view, we can see several objectives. The most important objective of the Chinese, IMHO, is not to rock the boat. China absolutely does not want a revolution in North Korea. One that would almost certainly result in millions of refugees entering China; one that would very likely result in the USA extending its military tentacles into a region adjacent to it; one that may well give the people of Hong Kong ideas the Chinese don't want them having (any more than they already do). China is quite happy to kick the proverbial can down the road. That's not to say that they're not irritated by NK, but as long as Kim Jong-Un doesn't push things too far -- as long as he doesn't do something so egregious that he forces "the west's" hands militarily, he will likely be given a pass by his Chinese patrons.
On the other hand, the west, and specifically the USA, is mired in internal conflict. For some of the reasons you named, but IMO also more importantly for structural reasons having to do with the US's long history of prosperity, it no longer has the will to enter anything other than token military conflicts. The same is even more true of Europe, who have essentially ceased to be players on the world stage. (Russia has, of course, recognised this reality and that is why, IMO, Putin's strategy is to essentially revert to history and become a mercenary country. Partly to as a method of retaining importance; in no small part to ensure China doesn't, with its growing power, get any ideas. The strategy makes sense, though I'm not sure it will work.)
Just to finish off the goals game: what should the US want? Most obviously, they want for NK to stop developing nuclear weapons; that much is obvious. As just mentioned, they don't want a military conflict, since it might lead to a confrontation with China which they are likely to lose badly. (It's about will, not about supposed technical superiority.) What else does the US want?
Well, a revolution in North Korea wouldn't be a terrible outcome, for the same reason that China doesn't want it. Ironically, one of the best strategies for doing that is to do nothing -- after all, Kim Song-un is playing this game for a reason: Trump is absolutely correct in pointing out that what he wants is extortion money. So: do nothing. For once, the constant bickering in western countries actually helps!
The big picture, IMO, is one of Chinese ascent and one of US -- and by extension, European -- decline. There's not much that can stop this from happening, as I see it. It is as inevitable as the ascent of America was in the 19th century. This period will be punctuated by heightened risks, as allegiances are realigned in light of the new hegemon. Kim Jong-un, to my mind, has inserted himself into this uncertainty with the goal of hanging on to power without effecting the reforms that would almost certainly see him thrown from power. By the way, we should consider the contribution of western "humanitarian" democracies to this logic. In addition to the conclusions you list for the worlds aspiring dictators, another lesson they will have learned from studying Libya and Egypt is that there's no point in currying favour, in trying to make deals, with western democracies. If there's a revolution, they will align, knee-jerk, behind the banner of "democracy" and "human rights", and the dictator will end up on the wrong side of a firing range.
I'll finish off where I began. Although the picture I've painted is bleak from the point of view of westerners, I don't think there's necessarily that much cause for alarm. The Chinese, from the evidence we have so far, appear to be doing a better job at ruling in their spheres of influence than we are. Look at Africa, look at Singapore (I know; it's not the Chinese in power, but the ruling class is effectively ethically Chinese). They are already ruling Asia, and they're not fucking it up like we are the Middle East. No matter who the hegemon is in 30 years, they will not tolerate some lunatic playing with atomic weapons. So my view is that nuclear weapons will still be around, but I don't necessarily think they will proliferate and become 'normal' weapons which see a lot of use.
[I realise that some of things I've written are controversial, and reflect my politics. I hope if people disagree with me then we can just to it in a kind of intellectual way and not shout at each other, ok?]