The issue I have with posts like this (and there are lots) is that people seem to believe there is one magical "THE problem."
The Bills have lots of problems. Yes, D is one of them. Its been a major factor in the Bills struggles this year. But the D being a problem doesn't prevent TT from being one too. They are not mutually exclusive.
I have not been happy with TT's play this season but I'd actually be okay with giving him one more year to see how he does with a healthy WR corps around him. The problem is they can't just give him one more year, they have to make a decision that negatively impacts their cap in a significant way for 3 seasons.
Has anybody seen enough from TT to justify a 3 year cap commitment? Hell, I'd rather slap the franchise tag on him and overpay him for one season that kill the cap for 3.