The Russians could care less about THAAD in South Korea. They are very upset about Aegis in Poland.
Given THAADs defended area footprint and the geographic location... I can’t see why Russia would give a flying rip about it. If they want to waste the resources to set up missile systems to “counter” THAAD ... I’d say that’s great. That is just that many missile systems they don’t have available to threaten someone else.