I'm pretty sure this issue has been pointed out many times in the past, so here is a quick summary.
1. Yes PLA can destroy THAAD easily, but that would mean actually physically attacking South Korea, which is something you don't want to do for the obvious reason.
2. THAAD's radar reach far out to Inner Mongolia, Manchuria and all the way to Russia and beyond, it can keep track and detect all of Chinese missile test launches and identify the real warhead from decoy, and this greatly decrease China's 2nd strike ability. And it can monitor Chinese airspace live time, this takes away Chinese strategic initiative in the event of war, as well as its ability to do 2nd strike.
3. Also send a political message to SK that you can't enjoys all the benefits from doing business with China while at same time violating Chinese interest.
so much "blowjobs" about THAAD from China before the deployment: if SK allows the deployment to go ahead, China will do this and this and this...and after the deployment, China has been looking for excuses to accept it quietly as usaul.
you have grossly over estimated China's capacity and will.