Nope, most successfully stopped attacks are secret. Nobody would even know.
The truth is that even a minor succesfully stopped attack they'd advertise till the end of time, to score political and press points.
I didn't say they're perfect methods or systems. Intelligence operations have mixed outcones partially dependent on adversarial actions such as ops sec or counterintel. This isn't a new or unique phenomenon.
The reason we can't see the justification is because it doesn't exist.
Here is a small list of the ones that were publicized. https://en.m.wikipedia.org/wiki/List_of_unsuccessful_terrori...
It doesn't really speak to or against your point. You're not going to justify an entire national security system on a subset of one of its many functions.
Yes, that is the primary problem with spy agencies - that they don't feel accountable. They SHOULD be.
The current paradigm is that agencies generally are accountable to congressional committees and the courts. I'm not sure how we would make them accountable to the public when there's very little accountability for elected representatives even.
... but of course it depends on the standard for what 'prevented' means. De-radicalized a potential terrorist, does that mean a terror attack was stopped/prevented.
It's quite complicated in practice, alas.