I've heard that Mack wasn't entirely pleased with the media and fans in general asking him at every press conference when he was going to retire and let Muschamp take over. By bringing in Diaz, no one is going to immediately assume he is the heir apparent.
Still, that's a lot of money to pay Diaz. With the incentives, that's not significantly less than what they were paying their "coach in waiting." I was shocked when I read that.