Yes you are giving them too much undeserved credit. You're asking us to have faith in a government agency being able to keep an enormous amount of digital data (and growing fast) when we've already seen that fail plenty of times.

So the NSA is special? What happens when the political winds change and they experience budget cutbacks, and some of the really talented employees move to the private sector? Or when they bring in outside private sector contractors.

The slip up doesn't even have to be monumental in itself, but the consequences are. The real terrorism (the one that will actually affect a large number of people) is and will be cyber based. You're basically stockpiling weapons.