I think turning down an opportunity you're interested out of concern about your "legacy" is an ego trip.
The guy is going to get paid a ton of money to coach football and it's a new test. I'm surprised it happened but we'll see. In reading his "organization bible" that he wants part of the contract, it sounds interesting.
From Newsweek, "Belichick's bible, which he has shared with other college and pro teams but customized specifically for the UNC position, contains "structure, payment plans, staffing choices and salary minimums position by positions" that "would require historic levels of investment from the school."
To appease Belichick, the school would also have to commit to hiring a coaching staff run by Belichick and a recruitment staff run by a sitting college GM who would need to be bought out of his current contract."