So the FBI would only be able to wiretap suspects who are either too dumb to use encryption — in which case they ought to be easy enough to catch without wiretaps
I think the author under-estimates the difficultly of catching criminals.
Agreed. The dumb ones will work out of 'the game' quickly enough and are quickly replaced.
The bigger problem is not 'catching' them, but prosecuting them effectively. This requires prioritization of limited resources based on severity of offense, availability of sufficient evidence, and difficulty of prosecution. The last is ugly because it's almost directly economic, in that the ability to afford legal counsel creates some level of inequality.
I think the author under-estimates the difficultly of catching criminals.