I think it's simple. If the culture shift didn't happen, Allen doesn't have the room to grow into who he is. If he went to a team in full dysfunction, the expectations would not likely allow him to take the time to grow into a super star.
For that McD gets credit. He makes the overall decisions, the coaching hires, the general flow. He knows the O is not his strong area, so he leaves his generals in charge of their platoons. He doesn't have that Reid/Mahomes relationship, but he has the calming presence to say, Allen - go be the best you!
Now he has some issues coaching, game day decisions mostly. But he has learned there over the years. I believe barring major injury; this is the year that the steady approach wins it all. We need the horses on D in the playoffs. Which means some great play by rookies and FA's.