He's not giving up his degree, he'll graduate in June. He has no academic reason to come back next year (ballroom dancing?). We're not talking about an underclassman giving up his possible future. He's put in his four years, and gave up a great deal of his mind and body to USC. He seemed to consider coming back, to benefit his own development and help the team, and said he was leaning that direction on multiple occasions. When it became clear his draft status would likely be much better this year than next, he jumped.
Hard to blame him. Matt Leinart stayed a year too long, didn't get the national championship he assumed, and lost out on being the number one pick the year before. San Francisco was very outspoken about Leinart being their top choice, but instead had to give Alex Smith the record breaking contract. Sanchez will need an impressive Combine performance to go that high, but it's not out of the question, especially with Detroit so obviously in need at the quarterback position. Add to that USC's depth at QB- no guarantee he'd start next year, and the possibility of injury added to that, by staying he could fall off the radar completely. Would 30 million sway any of your decisions?
The one who seems most likely to gain by his staying is Pete Carroll, which is why his "throwing Mark under the bus" comments about his unpreparedness seem so suspect. Pete doesn't coach for free, and his contract is very much tied to his success. Assumably there's much we don't know, agreements they may have had and details about off-field issues, but those comments to the press can't be seen as to Mark's benefit. How would it have been received if Sanchez suggested Carroll coach for free the year he would play for free (or less)? Pete Carroll is a smart coach who understands the media- applauding his "unfettered honesty" is naive.
Also worth noting, NFL teams have capable staffs who develop players -Matt Cassel is a great example. Don't assume he could only become a good quarterback at the next level if he gets more college experience. He's had four years of that, albeit injured and not starting for some. Whether he's successful in the NFL or not will depend on many other factors, such as what system he goes to, his own intellect and physicality, and maybe a little luck. Another year at USC is not the whole answer.
Hopefully the next time a kid gives Pete Carroll four good years, he'll temper some of his own wants with a little support. And class.