It may be as much about climbing a new mountain as padding his bank account. It's hard to imagine that he would ever duplicate the season he just had at LSU.
Unless he had some other influence to stay here, it's only natural that he would look for the next big challenge in his coaching career. Plus he will get that opportunity while working with a close friend. He will likely have a lot of leash to do his job. His moving on seemed pretty inevitable to me.