I believe the THAAD raders are put OUTSIDE the US bases in South Korea today.

I doubt the murrika and the SKoreans can stop Chinese troops taking the whole of the penisula if there is a hot war TODAY. This is not a 1950 China anymore. In this sense, murrika doesn't need SK for its hegemonic ambition-- except as a forward base for its long range radars looking into China. Taiwan would be more valuable to murrika for stopping China breaking out of the "First Island Chain."

Sn Tze said, "Do not assume the enemy will not come but rely on your own preparations to thwart his ambitions." NK is an ADDITIONAL buffer if SK refuses to play the game China's way. China canNOT guarantee that the SK will stay neutral even!