Football is a team sport. As a result it would be impossible to find any one single cause for a disappointing season.
There are several factors that contributed to the pathetic and underachieving season for the Steelers....injuries, coaching, turn-overs, penalities, solar flares, etc...but to say the Steelers sucked soley because of Mike Tomlin in unrealistic.
However as the head coach, he must shoulder some of the responsibility. Head coaches don't play on the field but they do prepare their players to play and coaches to coach. That preparation has some impact to the outcome of the game...After all the players still need to execute on the field.
In any case, there are several factors that can be used to determine a coaches effectiveness...Won/Loss record, team discipline, penalites, turn-overs, in-game adjustments, clock management, etc...everyone is going to have an off game here and there, but when certain trends continue on for an entire season or longer at some point the HC needs to be accountable.
There were definitely some disturbing trends this season and the team (including the coaches) need to share the burden.
If we are going to crown a HC as one of the greats after winning a couple of SBs, then we need to hold him responsible when times are tough....