[Aslan]

How many games did the Colts win last season? How many did they win this season? Isn't that an 7-8 win drop?

Even Manning and Brady have not been able to repeat a perfect regular season, so Cassel cannot be expected to be perfect. You say did not make the playoffs, while true, is a little skewed. How many teams win 11 games and do not make the playoffs? The last two seasons an 8-8 team and a 7-9 team made the playoffs. Almost any season a team with 11 wins, gets in. He played well enough to get in.

In most sports, 5 wins is not that big a difference.
In football, with just 16 regular season games, it's huge. A win or two drop off in the NFL is huge. It makes the difference between being the wildcard and going on a SB run, like the Packers last year, and potentially Giants this year. Or Sitting at the house watching. Five less wins usually means you have a top pick and need to reevaluate your situation.
Going from undefeated, in the SB, to not even making the playoffs is a big drop off. And it's because Cassel was QB, not Brady.