TL;DR: As much as a student would need to know in order to succeed in their career.
I think part of this answer will be subjective - does a student even need to master course material to pass, or do they need to be familiar enough with it to know how to re-learn it later in their career? I come from a STEM background - I know how to do calculus, but if you asked me to come up with a Taylor polynomial right now off the top of my head, I'd give you a blank stare.
In my opinion, the goal of any class is to teach familiarity with the subject at hand. No student will ever use 100% of what is taught in a given class - their career will dictate what they need to know. As they spend more and more time working in a particular field, the knowledge that is important will stay with them, and the knowledge that isn't important will be set aside - not entirely forgotten, but not readily accessible without review. The point is - they still know how to re-learn the material; they just don't know it off the top of their head.
With that in mind, I argue that the proportion of course material mastery that we should consider as "passing" is however much is needed to ensure that a student could, given a reasonable amount of preparation, re-learn the material at any point later in their career. That amount will vary from class to class, and it's worth remembering that some skills will carry over and be reinforced from class to class. To my mind, this means that fundamental skills - calculus, introductory classes, writing courses - need to be held to a higher standard.
As for incentives for professors - there's a variety. For one, if you want to work at a high-caliber institution, you're going to want high-caliber students. This means pushing them to learn and master more of the material than perhaps a professor at a not-so-high-caliber institution might care. For another, there's an element of peer pressure involved: if I teach my students poorly, and don't require them to learn enough of the material, then my peers will have to pick up the slack in later courses. That reflects poorly on my teaching, and it means that I may have to do the same thing with these students in later courses that I teach.
So what proportion of course material mastery should we consider is "passing?" Whatever is necessary for a student to (1) be able to relearn the material in a reasonable amount of time and (2) be able to retain the core concepts for use in later courses.