7 RBs on IR and they still had a decent running attack. They had Cutler at QB, who will improve, and Shanahan's knack for always having a good OL and running game regardless of the RB. Next season you keep the good QB and probably lose the OL and running game, while STILL not improving any more on defense than had Shanahan stayed. That makes absolutely no sense to me.
Shanahan was an offensive coordinator and Denver's offense was #2 in the league this season in total yards, which shows how well teams could coach against his "tendencies". Granted, their scoring was only middle of the league, but they had to go further on most drives since their defense was one of the worst in the league. They needed a new defensive coordinator and maybe someone else picking personnel for the defense, but there's no need for a new head coach. And firing him without giving him any inkling that it was coming? What good head coach would go there? That's messed up.