It's part and parcel. I don't think anyone has absolved the defense for their poor play the first half of the season. They absolutely sucked at third downs, which is noone's fault but their own. But dig deeper, since you referred to the "depth" of the game. As the game progresses, why are they horrendous at third downs? They are gassed. Why are they gassed? Because they cannot get off the field. Therefore, their only time to recover is for the offense to string together a series of first downs or, I don't know, maybe run some time off the damn clock.
If you look at our TOP, we actually possess the ball about a minute more than our opponents. Now someone who has never watched a Bills game can argue, "well, the offense is doing it's job." But actually watch the game, you understand (which equates to "knowledge") that the quality of the possessions matter as much as TOP. Dominating TOP in the fourth quarter of a game we are being blown out of means little. It's the quality of the possessions when our defense is gassed that matters. I put that on Gailey as much as I do on the offense for poorly executing in those times. If we give up a long drive, then go 3 and out or turn the ball over, followed by another long drive, the offense has to step up or the defense will get slaughtered. That's what has happened.
To say it's all on the defense is incorrect. To say it's all on the offense is also incorrect. However my initial statement of it being "blasphemy" to point out how the offense has contributed to the defensive woes was indeed correct for the exact reason I am writing this post, despite my limited depth and knowledge of the game.