Despite all the talk about Pete Carroll leaving USC because of the "pending punishments" the NCAA is going to lay down on the program, I feel as though the bigger reason for Carroll "going pro" is that he realizes that USC is about the enter a decline.
No program in the past forty years has been able to stay on top of the NCAA for a decade. The University of Miami fell off in the mid 1990's, and again in the mid 2000's. The University of Alabama floats in and out of contention every five to ten years. Currently the Universities of Michigan and Tennessee, and California Los Angeles are all in rebuilding eras, yet only ten years ago were national title contenders.
USC's run of greatness that began in 2002 isn't in danger of reaching UCLA status (otherwise known as irrelevance), but it is in danger of reaching mediocrity. Before Carroll's resignation and departure for Seattle, USC were struggling in recruiting for the first time since 2002. For most of the decade the Trojans were the best recruiting team in the nation. In the past several years Florida, Alabama, Tennessee, Lousiana State, Texas, and Oklahoma have caught up the Trojans, but have also surpassed USC as the top recruiting schools in the nation (signifying the South's return to dominance in College football).
Another problem that USC may run into comes from their own conference, the Pac 10. This year we saw USC fall from the crows nest and back to the decks. Pete Carroll is no longer the "only" coach in the conference, and Stanford, Oregon, Washington, and UCLA all look to be competitive for years to come; Oregon State, Arizona, and Cal Berkley will also be competitive as usual.
Pete Carroll is a smart man, and he has unfinished business left at the professional level of the sport. With a conference that is evening out, and an FBS that is returning to it's southern roots, Carroll understands that his pro stock can only go down. Younger coaches like Urban Meyer and Jim Harbaugh have already caught up (and in Meyer's case likely surpassed) to Carroll's professional value, and with both coaches likely to go pro before 2012, I think Carroll understands just how valuable the Seahawks job really is.
Carroll is also falling into an excellent situation. The Seahawks ownership has always been very committed to building a great team, and their fan base always ranks amidst the best in the NFL. The ownership has forfeited the franchises presidency over to Carroll who can use his decade of excellence in recruiting and coaching at the highest level of collegiate football towards developing an NFL roster.
The NFC West is also the best place a coach looking to rebuild his professional image can go. The Rams are at least two years away from seriously competing, the Cardinals are a Kurt Warner retirement away from falling back to mediocrity, and it's unknown as to whether or not the 49ers will ever "get it together" with this roster. Carroll is going to have a lot of rebuilding to do in the next two years, fortunately for him so will the entire NFC West.
As for USC, their best case scenario will be landing a coach like Jack Del Rio. Del Rio is what Carroll was ten years ago. An average NFL head coach, his mentality will translate incredibly to the college game. He is a USC graduate and a successful pro as a Cowboy. He'll have sway in Texas, California, and Florida. Yikes. The Jacksonville Jaguars are moving on without Jack Del Rio, that seems eminent, the Trojans should move forward with him.
The appeal of Southern California to college recruits will never go away. The average kid in Iowa, San Antonio, or western Pennsylvania doesn't understand the USC isn't on the beach. They just think Palm Trees, warmth, and prime time. In terms of "i'll go there for three years, have fun, then go pro" schools, USC is right up there with Miami as the place to go. The academics at USC are absolutely legitimate, but we all know that means nothing to high school players. Still, USC will stay strong in the FBS, and I do not see them falling out of bowl eligibility anytime in the next five to seven years.
But with Carroll going to Seattle, it has become obvious that USC's era of greatness in the 00's has come to an end, but Carroll's Trojans will forever remain the program of the decade.