The claim that professors neglect their teaching is always phrased as an econ-101 style argument about incentives, but I've never seen much evidence for it in practice. Quite the contrary - the social pressure of interacting with a class prompts most instructors to put in far more effort than the incentives demand.

Frankly, few people have the balls or sociopathic instincts to stand in front of dozens of people every day and tolerate looking like an incompetent, lazy ass.