This is silly. It is impossible to know how many other bombings have been stopped because of intelligence gathering attempts. In fact since 9/11 I've been extremely surprised at the low number of terrorist acts which have subsequently occurred, although maybe I was just wrong to assume how frequent / easy they would be to carry out.
There are much better reasons to oppose this than "To me it looks like they didn't stop every attack so it is useless".
Your corrective is well taken. The "Didn't work, did it!" response obscures the real dangers. These enthusiasts for blanket surveillance should be asked to consider "Who watches the watchers?". One answer is: you never know.
There are much better reasons to oppose this than "To me it looks like they didn't stop every attack so it is useless".