I think there are many excuses, but the biggest and most reasonable issue is proliferation. North Korea has sold not only its ballistic missile technology, not only chemical weapons technology, but nuclear weapons technology. See, e.g., https://www.scientificamerican.com/article/will-north-korea-...

Now, you might argue that it only does this because of sanctions. But NK has repeatedly eschewed normalization in favor of pursuing it's weapons programs. The examples of nuclear proliferation in the article above occurred _before_ sanctions were even imposed.

The problem is the regime: normalization of economic ties would result in the downfall of the Kim regime. The entire social and political structure of the country is built around the militarist cult of personality. The country has pursued the same path for over 50 years, 3 rulers, and the (de jure or de facto) transition of every other Communist and authoritarian regime from the post-WWII era. It's not a coincidence that is has, singularly, survived. And it's totally naive to think the regime would change now. The regime has persisted only because of it's radical rejection of normalization with the world.

That rejection of normalization doesn't mean a rejection of all international economic ties. But for various reasons the safest trade is black market trade as the scope is more easily controlled from the top.

That said, I think it's reasonable to accept a nuclear NK, as long as you're honest about the nuclear proliferation risk. Believing that NK could keep nuclear weapons without proliferation of nuclear tech would be willful denial. A realistic scenario is NK selling nuclear tech with the West expending resources to take it off the streets again, like the War on Drugs. Yet like the War on Drugs the result will be a lot of death as the U.S. often prefers using bullets to money, especially internationally. Over years and decades it will hasten the relative decline of American power as the perpetual conflict exposes the U.S. as ineffectual. China, like now, just sits on the sidelines watching the U.S. burn through political capital faster than cheap Chinese imports. (Which is one of several reasons China isn't eager to see either NK regime change or even the removal of nuclear weapons from NK. Interests in China and, especially, Russia see significant value in the pressures the NK problem imposes on the U.S.)

And that scenario excludes the issue of Japan remilitarizing. I think that aspect is overemphasized, partly because it's easier to articulate than it is to articulate the increase in amorphous geopolitical conflict that would be caused (and has been caused) by the steady supply of NK arms, including WMD arms, in the global black market.

In short, a nuclear NK inevitably means nuclear proliferation because the regime favors (and must favor) black market trade to sustain itself. Depending on your calculus, that could easily be the least worst scenario. But by no means is it a nice outcome, and it's not the kind of strategic compromise Americans are amenable to, especially in modern times.

IMO, the path with the least violence is Chinese engineered regime change. Say, Kim Jong-un disappearing on a trip to Beijing. But that's the most difficult outcome to achieve because it requires Americans not only being realistic about the alternatives, but also making some very hard and (most importantly for domestic politics) explicit strategic compromises with China in the Pacific. I imagine the U.S. would have to be prepared, at least in principle, to capitulate on Taiwan or the South China Sea (or both), and then hope negotiators could secure a less costly quid pro quo.

FWIW, I think a lot of countries, including some European countries, are prepared to accept a nuclear NK today, either formally or informally. But there is no tidy resolution to the problem of proliferation except to either deny or ignore it. It's easier to ignore if you're not the U.S., because everybody around the world has basically relied on the U.S. to stop and punish proliferation. If you remember the Syrian chemical attacks, the British, French, and Germans excoriated Obama for not retaliating, yet none were prepared to retaliate themselves.[1]

[1] That's another example where Obama's (belated) strategic clarity had the unintended consequence of exposing something unspeakable about the global order.