In my opinion, the US should make it clear to China that if North Korea gets absorbed by the South, as long as China is not threatening Korea, the US sees no need for US forces anywhere in the country, let along north of the current line.
If US withdraws from Korea, it's forfeiting it's sphere of influence in the region. Will it happen one day? Of course. But until that day comes, the US will not budge until China too is conquered (unlikely).