The nation is far more likely to fall because of creeping totalitarianism because of this kind of massive surveillance than from lack of massive surveillance failing to detect a terrorist attack. These programs were in place and they failed to stop the Boston bombers when the feds had a lead from the Russians to put them on their radar. You raise a false choice.
However it's important to remember that one the intelligence is collected and shared it's then in the hands of men to ACT on it. The intellegence was there...men refused to act in the case of the Boston Bomber.
Regardless of how sophisticated our machinery is...it requires human action..and humans will never be perfect this side of heaven.