My first guess, when I posed the original question, was organized crime. People who do organized crime would be the ones to benefit the most from encrypted phones. But anecdotally, it seems to me that organized crime is down in the U.S. too. I might be wrong, that's why I asked for evidence.
The white collar crime, yeah, some forms of it might benefit from encrypted communication. But again, where is the evidence? Is the tax evasion (non-institutionalized) skyrocketing?
The whole claim is just very suspicious. I think there is so much open data today that even with encrypted phones it's much harder to leave less trace than in the past.