On the other hand, there have been numerous attempted terrorist attacks on planes in the United States. All of them have been foiled without assistance from TSA

On the other hand, there have been numerous attempted terrorist attacks on planes in the United States. All of them have been foiled without assistance from TSA, even the ones that weren't stopped until the would-be terrorist was on the airplane. This isn't proof that TSA hasn't stopped any attacks. You could be correct that fear of the TSA has stopped terrorists from trying, or trying as often. However, given that there have been actual attempts and TSA has failed to foil any of them, it seems doubtful.