The Eagles didn't just move on for not winning a championship with Reid-- I think a lot of people forget the circumstances there.
They moved on after they spent big in a 2011 off-season where they were called the "dream team", and then proceeded to miss the playoffs and win only 8 games. This was followed by a 2012 season which was a complete implosion: they won 4 games that year and finished with two QB's on their active roster: a washed up Mike Vick and... Trent Edwards. Most coaches no matter how good they are, will be fired after two seasons of that nature when the QB situation is in flux and you're prime for a rebuild.
The Bills will always be likely to keep McDermott if he maintains a stranglehold on the division with double digit wins and Josh as a healthy active starter, even if the playoff success is middling. But if we have a disaster-type season like the 2012 Eagles did? I think it shouldn't even be a question to move on.