The trouble is that argument is a spiral. The government might pretend they can't access signal so people will use signal and they can snoop with it.

On the other hand they might be happy with the suspicion they can access signal because they can't, and they don't want people to use it.

And that spiral is never ending. Unless you're only ever going to talk freely with people who are naked and submerged in the sea, you have to trust someone.