Except you are forgetting his ego/legacy. I don't think he cares about the money at all - he can make money doing whatever he wants.
But if he quits now, he won't be considered a failure at all. He took over the dog shittiest of programs and got them "competing" at the national level almost overnight. But now he won't have his generational talent at WR/CB or his son at QB, who, despite the head case that he is, was a legitimate talent at QB.
He will not be able to sustain the success he has had. He needs to get out while he is "on top."