There's however the documented cases of the guns carried onboard that they did not prevent. And there's documented evidence of TSA confiscating butter knife from the pilot (obviously, he was going to hijack himself), purse with picture of a gun from a 17-year girl (that was undoubtedly Hermione Granger and TSA knew she could turn picture of a gun to a real gun with her magic), plastic lightsaber from a small kid (Anakin Skywalker, I presume, and everybody knows how he turned out, so better to be safe) and a plastic 4 inch GI Joe gun from another kid (again, who knows those underage magicians - maybe they could turn it to the real one). I have no idea why they have problems proving they prevent many acts of terror, with such record. People must be stupid.
Yep, imagine the horrors that would have fallen upon us if a terrist had gotten through with some nail clippers.
Many security experts think it is trivial to get weaponry past them. They aren't extensively trained, you know, they are cheap rental guards following a procedure.
So in fact I would say the TSA has stopped nothing, the only prevention has come from the FBI and CIA, locking the cockpit door, and putting air marshals on the plane.
That's the point, right, that you aren't in a position to guess, and it's slimy position to be put in. It's the same way ADT can sell millions of home alarms every year - there's just no way to prove how many events a reactive system stops.