I don't think Bruce Arians is a stupid man at all. I think he is arrogant and overconfident and those characteristics extended themselves to his offense. I think a large part of the entitlement and complacency on the Steelers stems from the offensive side of the ball.
Firing Ray Horton was absolutely necessary and a smart move. You do not want a disgruntled employee on your staff. Horton was reportedly livid about not being hired as head coach- you can't have that attitude on a team.
The problem isn't firing Horton it is hiring the 31st ranked defense's coordinator, Todd Bowles. Bowles took over for Juan Castillo in mid-October so I don't know how much he is responsible for the Eagles failure. The Eagles did go from 10th in scoring defense to 29th in scoring defense between 2011 and 2012 though.
Did Horton really think he was going to be the face of the Cardinals franchise when he looks like a Kenny Powers impersonator though?