MM really isn't much different than other NFL coaches. For the most part they all do things by the same book, and often their destinies are determined by things out of their control. Maybe half their team gets injured, maybe an opposing player has an unbelievable game, a fluke fumble here and there wins and/or loses games. These things tend to even out over a coaches career (if it lasts long enough), and that coaches future is sometimes based on these fluke occurances, for better or worse.
After 5 years MM's successes and failures can no longer be attributed to flukes, even if they are the cause. That sounds contradictory, but it's not. Ultimately the head coach must take the blame for the overall direction of the organization. While it's hard (but not impossible) to blame injuries on the coaching staff, I think it's obvious that there are things going on in the franchise that could and should be fixed. For example, and you can go back to the Holmgren days on this one, the Packers are notorious for not squashing opponents and putting them away. This is the NFL, not college. You don't scare the other team and their talent level is probably very close to yours no matter who you are playing. Write this down, and watch it for yourself: Every time the Packers have a 7 point or more lead, and a 3rd down comes up, they will run the ball, throw a screen, or throw a slant short of the first down. EVERY. TIME. That is the coach effectively telling his players that he does not trust them to not turn the ball over. You have one of the greatest young QB's in the league and easily the best receiving core, but MM does not use their potential. After the other team comes back and takes the lead you see the Packers scrambling to try to turn it back on. Remember all those fourth quarter Favre INT's? Many of those were made in a desperate move to try to take back the momentum.
MM has to go for the same reason Ned Yost was canned. You can't waste young talent even if it's ultimately not entirely your fault.