How long do you think he'd wait for Mack Brown to vacate the position? I'd imagine Muschamp will leave for a head coach offer before Mack leaves UT. He was in the running for the Arkansas head job this year. With Major, I don't see him leaving Austin unless an unbelievable HC offer came up. I could see him taking over for Greg Davis as the OC/playcaller within a couple years, and then spending 2-3 years doing that. By then, Mack may be about ready to step aside and Major could take over.
You're probably right. Sometimes I feel like Mack Brown might coach for another decade or two, and sometimes I think he's only one more really good team away from deciding to retire and move into the athletic department. Muschamp is making an awful lot of money for a defensive coordinator, and he did make only a slightly vertical move coming to Texas. I think one of the things that might make that move really easy to explain(besides the enormous salary jump) is if he was told that he would have first crack at the job if Mack steps down.
The last 2 defensive coordinators for UT went on to get a Head Coaching job after 1-2 seasons (Greg Robinson left after 1 and Chizik left after 2). For a young and up and coming coach like Mushcamp, this move maybe the best stepping stone for him to get a good HC offer. The fact that there was a big pay increase involved and that he would probably get a better offer after coaching at UT (against lesser competition compared to the SEC-a different discussion) is enough to make the move IMO.