Defenses adapt given enough time to stop offenses. I think the K-gun was on top for about 2 years until D's caught up. My own opinion was to drop 7-8 to defend the pass, and make Buffalo beat you with the run. Yes, TT put up a lot of yards. But I think Kelly preferred to pass first and didn't have the patience to have 15 play drives mostly with the run. As proof see SB25. Lastly, Kelly had a great grasp of the game to be able to make all his own calls. But when up against the opposing team's entire coaching staff, making their own calls, he was out-numbered and out-planned