There aren't that many steps behind them to move backwards. The worst team in the league finished only two games behind the Bills.
I think Donahoe is a pretty good football guy but the bottom line is he didn't get the job done. Injuries, salary cap "jail", an owner than meddled, are (perhaps) legitimate reasons (or excuses depending on how one looks at it). But the bottom line is the Bills haven't been successful during TD's entire reign here. At some point he has to be accountable.
Question: how much longer would you have given him to turn things around?
All that being said if Marv is the choice to GM this team I'd have preferred TD. Firing a guy just for the sake of firing him doesn't do it for me. Firing him only makes sense if you hire a capable replacement.