More than half of my engineering curriculum was taught by prolific researchers who couldn't teach worth a damn. I was a tutor through most of college and found myself "reteaching" a lot of the stuff they would teach to others who came looking for help. Not because I was bright, see I struggled to understand the same topics, but I was able to break the topics down in a way that made more sense. Tying "building block" concepts progressively, until the process showed the complete picture, at which point I could teach them to myself for my own understanding, and then to others. That's when I realized good teachers require the whole package of skills; proficiency in their subject and a mind to educate by facilitating the process of connecting concepts.
Sounds like a good place for a free market to open up. What teaching is worth should lean heavily on a feedback/review framework like Amazon's such that people don't end up paying for a class that sucks, by every student's experience, because the professor can't communicate concepts, or communicate at all. Like the time I spent almost weeks trying to figure out what the foreigner in my Space Systems course meant by "papamaaa". By the way, that's "performance".