We have no idea how many attacks have been prevented (everyone likes to claim this is zero....but they do so without any evidence to support that, as relevant information is surely classified beyond Top Secret).
We have only the slightest, tiniest, smallest idea of what the NSA and other intelligence organizations are capable of.
British intelligence took how long to tell people that they cracked enigma? 50 years?
Out of the THOUSANDS of documents that Edward Snowden leaked to reporters, the public has access to what? on the order of 50?
Yeah....we know nothing...so saying one way or the other is kind of ridiculous at this point.