Yahoo! Sports had an intriguing take on this - (LINK)
It touches a bit on an important cultural difference between the college game and the pro game, which would be that the NFL is a players' league where coaches aren't often quite as celebrated, with the college game typically venerating coaches a little more. That's true, I would say.
Essentially, it asks the question - "Does Kelly realize what he may be giving up?". In the NFL, you wouldn't necessarily hear the names John Fox and Mike McCarthy before you heard the names Peyton Manning and Aaron Rodgers. Coaches that have won, or at least gotten to the BCS, tend to be held in somewhat higher esteem overall.
I do wonder if that is something he would think about - if he jumped to the NFL, his success would be dependent on many more things outside his control, and he does have at least a little more control over his own legacy at Notre Dame, in my estimation.