Not sure what the current agreements are for US forces in Korea, but I wouldn't think a new president could single handedly stop or have THAAD removed from S Korea. Or at least right away. That is assuming that THAAD is installed on US military installations there.
I also wonder many times on reports of protests and if they are actually the very small minority. Take for instance those in Okinawa who want the US forces out. Yes, no doubt there are Okinawans who want ALL US forces out of Okinawa. That is for many reasons, including Okinawa feeling treated like a bastard step-child. That said, it is my understanding that many Okinawans quietly or even openly embrace the US being there. There are thousands of Okinawans employed by the US and it also brings money into the economy. I haven't been there since around 2012, but that was when the pressure to get US forces out of Okinawa was very high. Now that things are in motion to relocate a lot of them off Okinawa, it seems like that has died down. It will still be a while before the bulk of the forces are relocated, but will see how the mood changes once the money flow into the local economy goes way down.