Astute point you're making that if they're this concerned about additional whistleblowers, then there must be a lot out there that is worse than what we've heard so far. This isn't just about wiretapping, because that aspect couldn't be worse than we've heard so far.
Well, Snowden alleges warrantless wiretapping, but didn't have evidence of that. [1] A subsequent whistle-blower bringing proof that it wasn't just possible, but that it was done (routinely), would be worse. Particularly if they had proof of warrantless wiretaps done for bureaucratic/political reasons. [2]
Similarly, if there was proof that no real effort is being expended to identify citizen data vs non-citizen data, it would be 'worse'.
At least in the eyes of those who are following the traditional media account and are still giving the government the benefit of the doubt and buying their defenses. [3]
[1] At least he hadn't presented any, thus far. Just a document outlining the technical capability of bulk copy and 'intercept' existed and his assertion that he could initiate those intercepts without any oversight, let alone legal basis.
[2] Which is something alleged by another whistle-blower, but again, without iron-clad proof. Which is apparently what you absolutely have to have, for the mainstream media to pay any attention to the story. (And even then, they'll probably just talk about you, personally.)
[3] Despite those defenses having to be walked back time and again for containing falsehoods and no corrected defenses being offered.
