From the Chinese point of view, North Korea served the same function in the 1990s as it did in 1950: It was a buffer zone between the now economically powerful South Koreans (and the U.S. military) and Manchuria. The Russians were, as during the Korean War, interested in but not obsessed by the Korean situation, the more so as Russia shifted most of its attention west. The United States was concerned that a collapse in North Korea would trigger tensions with the Chinese and undermine the stability of its ally, South Korea. And the South Koreans were hesitant to undertake any actions that might trigger a response from North Korean artillery within range of Seoul, where a large portion of South Korea's population, government, industry and financial interests reside. In addition, they were concerned that a collapsing North would create a massive economic crisis in the South, having watched the difficulties of German integration and recognizing the even wider economic and social gap between the two Koreas.
In a real sense, no one outside of North Korea was interested in changing the borders of the Peninsula. The same calculations that had created the division in the first place and maintained it during and after the Korean War remained intact. Everyone either had a reason to want to maintain an independent North Korea (even with a communist regime) or were not eager risk a change in the status quo.
The most difficult question to answer is not how the United States viewed the potential destabilization of North Korea but rather its willingness to maintain a significant troop level in South Korea. The reason for intervening in the first place was murky. The U.S. military presence between 1953 and 1991 was intended to maintain the status quo during the Cold War. The willingness to remain beyond that is more complex.
Part of it simply had to do with inertia. Just as U.S. troops remain in Germany a generation after the end of the Cold War, it was easier not to reconsider U.S. strategy in Korea than to endure the internal stress of reconsidering it. Obviously, the United States did not want tensions between South Korea and North Korea, or to have the North Koreans misunderstand a withdrawal as an invitation to try another military move on the South, however unlikely. The Japanese saw Korean unification as problematic to their interests, since it could create a nearby industrial economic power of more than 70 million people and rekindle old rivalries. And North Korea, it would seem, actually welcomes the American presence, believing it limits South Korean adventurism. Between inertia and what we will call a proto-strategy, the United States remains.
With the loss of its Cold War patrons and the changing dynamic of the post-Cold War world, the North Koreans developed a survival strategy that Stratfor identified in the 1990s. The Koreans' intention was to appear -- simultaneously -- weak, fearsome and crazy. This was not an easy strategy to carry out, but they have carried it out well. First, they made certain that they were perceived to be always on the verge of internal collapse and thus not a direct threat to anyone but themselves. They went out of their way to emphasize their economic problems, particularly the famines in the 1990s. They wanted no one to think they were intent on being an aggressor unless provoked severely.
Second, they wanted to appear to be fearsome. This would at first blush seem to contradict the impression of weakness, but they managed it brilliantly by perpetually reminding the world that they were close to developing nuclear weapons and longer-range missiles. Recognizing that the Americans and Japanese had a reflexive obsession with nuclear weapons, Pyongyang constantly made it appear that they were capable of developing nuclear weapons but were not yet there. Not being there yet meant that no one had to do something about the weapons. Being close to developing them meant that it was dangerous to provoke them. Even North Korea's two nuclear tests have, intentionally or incidentally, appeared sub-par, leaving its neighbors able to doubt the technological prowess of the "Hermit Kingdom."