Yes:
[There] is nothing inherently wrong with making adjustments to US forces in South Korea based on the evolution of the North Korean threat, changes in military technology, opportunities to rationalize and streamline command and control arrangements, improvements in South Korean military conventional capabilities, or the need to meet other competing demands (as the Bush administration did in 2004 when it re-deployed 10,000 troops from South Korea to Iraq during the second Gulf War). Changes in the US military presence in Europe and Japan have been made over the years in response to such factors. Nor is there anything sacrosanct about the number 28,500—an arbitrary figure arrived at without any systematic assessment of operational requirements that took on a life of its own.
The actual defense of South Korea does not depend on these forces, but rather on the capacity to rapidly reinforce the South with additional ground troops, air and naval forces from the US and elsewhere in the region. It is doubtful that North Korea would be any less deterred if the US made modest reductions in its troop presence as long as enough capability remained in place to protect and support those reinforcements. [emphasis added]
The author also notes that America has other interests that South Korea-based forces defend--subsidized by South Korea:
[The] US alliance with South Korea embodies more than just a defense commitment to that country; it is also a pillar of the US position in the Asia-Pacific region and central to preserving a favorable balance of power with China. The US forces based there are critical to the defense of Japan and could be used to project US military force more broadly in the region if circumstances warranted. A rapid and unilateral pullout would further damage whatever is left of the credibility of US security commitments in the region. [emphasis added]
I've noted our other interests and have suggested a force evolution that would look good for North Korea yet retain our ability to funnel reinforcements to South Korea in a crisis.
A deal with North Korea on nukes and eventually conventional forces would make the North Korea issue much better; but it does not end our security problems in the region.
And really, if America and North Korea can make a true peace (and I wouldn't bet on it, but things remain the same until they don't), North Korea may see China as the historic threat it has long been and welcome an American presence in South Korea.
As a partial aside, I don't think North Korea has such a large (but poorly trained and equipped by now, and even hungry) army to threaten South Korea as much as it has a large army to keep those potential rebels from threatening North Korea. While North Korea in the long run should want to divert resources to civilian purposes, in the short run letting large numbers of young men remain in the civilian world outside of tight army control is a security challenge.
