The great artists couldn't bring themselves to produce shit, any more than great people in any field ever can. Show me an example of shit made by Leonardo or Van Eyck or Brueghel.
It's very well documented that rewards diminish both intrinsic motivation and creativity
In studies where they offer one group of undergrads money to complete some random task and another group does it for free? Things work differently at the other end of the expertise spectrum.
The studies are pretty wide ranging. For example, people paid to wear their seatbelts are less likely to do so. People who are offered money if they lose weight are less successful, and are more likely to gain even more weight. If teachers are offered money if their students improve on a test, the students will do worse on the test. Students who study something in an attempt to get a good grade will have a more superficial understanding of the material than they would otherwise.
"Things work differently at the other end of the expertise spectrum."
Maybe that's true once your work is at the genius level and you don't need any more external validation, but even if so how do you get there? If you've been mastering your craft for twenty years and then you're offered money it might not be damaging, if done right, but being offered money too soon may well harm the average person.
"The great artists couldn't bring themselves to produce shit, any more than great people in any field ever can."
George Lucas.
"George Lucas"
I think that George Lucas has very aptly demonstrated that he is not a great artist. He got lucky with Star Wars. With his next two movies he had the sense to let someone else direct. Then he let the success go to his head.
That last sentence is extremely disingenuous.
I would be like to read the his original argument though.
That said, I think the point stands, and that's why there's not a strong correlation between greatness in the arts and salary. The great artistic minds, even when producing stuff that I feel like was below par for their work, were doing so out of a genuine desire to do something important, even if they didn't have a perfect batting average. Similarly, those who were driven by that desire didn't cave just because they weren't commercial successes.
A) The psychological effect of working for a church/patron is probably not analogous to, say, trying to produce a blog post that makes the front page of Digg. To borrow from Herzberg's two-factor theory, their patronage was more of a hygiene factor than a motivation factor.
B) You're only looking at the best pieces from the best artists. It's likely that the average artist was producing mostly shit in hopes of getting paid. In fact probably even the best artists were producing mostly shit in hopes of getting paid.