For all of their occasional provocations, the North Koreans have been careful never to cross a line with conventional or nuclear power to compel a response from the South or the United States. Their ability to calibrate their provocations has been striking, even as their actions have escalated through nuclear tests to military action against South Korean ships and islands in the West Sea. Also striking is the manner in which those provocations have increased China's leverage with the United States.
The Difficulty of Extrication
At this point, it would be difficult for the United States to withdraw from South Korea. The North Korean nuclear threat fixes the situation in place, even for troops that aren't relevant to that threat. The troops could be withdrawn, but they won't be because the inertia of the situation makes it easier to leave them there than withdraw. As for the South Koreans, they simultaneously dislike the American presence and want it there, since it ensures U.S. military involvement in any crisis.
While the U.S. troop presence in Korea may not make the most sense in a global U.S. military strategy, it ironically seems to fit, at least for now, the interests of the Chinese, South Koreans and Japanese, and even in some sense the North Koreans. The United States, as the global power, therefore is locked into a deployment that does not match the regional requirements, requires endless explanation and is the source of frequent political complications. What we are left with is a U.S. strategy not based necessarily on the current situation but one tied to a historical legacy, left in place by inertia and held in place by the North Korean nuclear "threat."