That doesn't seem to be true. There are many court cases involving criminal conspiracies where you cannot find unrelated information about the involved people.
Due to the severe corruption of our institutions, the investigators in this case are the public. A time period of a decade is more than enough time to recall all the HUMINT assets that might be damaged by such disclosure. And frankly the blame for any forced transparency should fall squarely on the shoulders of the NSA for having bundled so much under one blanket of secrecy.
Like how NSA collects a shit ton of data on citizens... the vast majority of which has absolutely nothing to do with any kind of wrongdoing.
I'm only pointing this out because your comment has a negative tone towards what Snowden did.