This is all very true. I think in general teams fire coaches way way too soon. Often times the problems are more about the roster talent and especially the QB. You have these guys in Cleveland, Jacksonville, Washington getting canned every 2-3 years like clockwork, as though any coach would have made them winners. Then you have guys winning coach of the year and going 5-11 the next season because they lost the close games and ran out of luck this time around.
The Bengals, I think, realized a lot this and didn't cave to the pressure, as they held onto Marvin Lewis despite a string of disappointing seasons. That's a good example of how coaches are typically going to win as many games as the roster allows and also an example of why it's good to keep a coach long-term, as you don't have to change your system and schemes all the time either.
I'm not saying there aren't good and bad coaches out there, just that teams are too quick in giving up when unrealistic expectations aren't achieved immediately. The cost of starting over again is brutal for any football team, but it is spun to the fan base as a new hope. Nothing gets fans excited like scapegoating a coach who had no chance and bringing in the next guy, a new face with lots of promise. Most fans don't realize that hitting the reset button and changing the style of the team is going to set them back years.