There is a difference between a good coach and a cheerleader. Sean Payton, Bill Belichek, Harbaugh Brothers, are good coaches. You can count 1-2 wins per season that they don't win if they are not the coach.
Then you decent coaches, Coughlin, Tomlin, Mike Smith, Carrol. These are teams that wouldn't probably be effected either way if they lost their coach. Wouldn't change much.
Then you have coaches like Garret, Andy Reid, Norv Turner, Jeff Fisher, whose teams would probably be 1 or 2 wins more if they had a different coach. Once you accept that those coaches(notice most of them have long careers, because they do JuST enough to keep a job, or those 1 or 2 games don't mean much in the grand scheme of things) exist, you will understand the difference between a cheerleader and a coach.
The problem Garret has, is he doesn't have that same Pedigree as those other coaches(multiple division titles, conference/nfl championship appearances, etc. He is one or two games away from making headway into the playoffs, problem is, that's those 1-2 games he costs you!