My only assumption that would need citation (that I cannot give, I admit) is that currently the proportion of shady users is much higher on Tor than on the general Internet. From here, it's pure and simple logic: as more users transition to Tor (asymptotically all of them), this proportion can only decrease.
1. there's no clear indication whether the Tor userbase grows. Indeed, I would suspect the constant mobile is a hindrance
2. There's no clear indication in which direction the online CP people grow. They can be more or less over time. I am unaware of any studies because it's obviously extremely hard to measure.