I think you would have to see very bad season, less than 4 wins, without huge injury losses to explain the season. I think a solid season keeps him here, the team seems to have too much potential and chemistry to go back into rebuilding mode under a new HC. Do it oldschool, give your coach a chance to really settle in, very few teams that swap out coaches every few years are successful.
36-26 overall record in 4 years, one championship game appearance, a crazy crazy loss in last years playoffs on an even crazier play. I dont think these are the sort of numbers that get a guy ran out of town.
Its funny to see so many people comment that he isn't aggressive enough, because to me, comparing him to Sherman, the thing i notice most is that the guy has some balls. Sure, he's not the gutsiest guy in the league, but he's far far from the most conservative either. What you guys are saying about "playing not to lose" 95% of coaches in the NFL do the same.
Edit: Oh, you could also make a decent case that MM was one of the biggest contributing factors into turning BF's career around, once he started buying into Mike's system, he had one of the best years of his career in 2007.