For certain programs, being very good isn't good enough. I'm old enough to remember Earle Bruce and John Cooper getting bounced at tOSU and, more recently, Bo Pelini get shown the door at Nebraska. I get there were gripes about Pelini's behavior that helped lead to his dismissal, but all three of these guys had winning percentages above 70% (close to 80% for Bruce and Cooper), so the margin for error at programs like these (and Michigan's with Lloyd Carr as the obvious example) is much smaller than at other programs.