This assumption (i. e. that everything bad is done anyway, only in secret) is a truism. People who buy into this sort of cynicism will rely on the non-existence of evidence as evidence of their theory of all-powerful government able to suppress all evidence.

The idea is also destructive for any effort to improve matters: if everything and everyone is corrupt, there's no point in trying to hold anyone to account. And if those in power continually get hit with the wildest accusations people can think of, they will at some point abandon the idea of trying to do a good job and/or being honest because there are no incentives whatsoever when the discourse is dominated by narratives that are no longer in any way tied to their actual behaviour.