I don't think that many people realize how much Allen improved in his 2nd season, before Diggs, over his rookie one. The increase in his stats was dramatic. Allen was never viewed as an NFL ready QB and it showed, but his raw talent and athleticism improved his performance once he got those first 20 games in.
We're witnessing the opposite with Mac. His lack of raw talent and athleticism has outweighed his positives (decision making). He just can't make those tight window throws with enough zip that are needed when facing NFL caliber DBs. I believe he's lost confidence. Sure, the coaching situation hasn't helped at all, but to attribute 90% blame on them for his regression is just nonsense. We saw the beggining of this at the end of last season, when there weren't these excuses. Teams just don't fear him and it shows. On top of that, his theatrics are out of control. He's too emotional.