There is a difference between a foiled attack and one that is prevented.
For example if you lock your bike and when you come back, you see bolt cutter marks on your lock and your bike it still there, you know that it stopped the thief.
But for one attempt like that, there are probably a hundred more potential thieves that just passed by, saw your lock and didn't do anything.
Seeing your lock untouched doesn't mean it is useless. It may or may not have prevented your bike from being stolen, but there is no way to be sure.
In the same way, a terrorist may have had plans, but because of surveillance, he knew he would never have succeeded, and gave up. Seriously I doubt it, but that's a possibility.
For example if you lock your bike and when you come back, you see bolt cutter marks on your lock and your bike it still there, you know that it stopped the thief.
But for one attempt like that, there are probably a hundred more potential thieves that just passed by, saw your lock and didn't do anything.
Seeing your lock untouched doesn't mean it is useless. It may or may not have prevented your bike from being stolen, but there is no way to be sure.
In the same way, a terrorist may have had plans, but because of surveillance, he knew he would never have succeeded, and gave up. Seriously I doubt it, but that's a possibility.