Ability wise, i don't disagree. I don't doubt his ability for one second. He doesn't just magically lose the ability he had at Juve or what he displays regularly even today for the NT.
With that being said, he's been at Utd for 5 years. Its really difficult for me to accept any more excuses and just say you can't judge him at United. He's been surrounded by all kinds of players, he's played all across midfield and other than the fact that he just hasn't performed consistently, he also makes too many mistakes in midfield that lead to goals. A player's career is short, and you just cannot excuse 5 years just like that IMHO.
Now, for me as someone who enjoys Utds demise, I love that he doesn't deliver for them and consistenly does for the French NT
Also, I don't think its impossible that if he goes back to Juve or goes to another team he might go back to being a top midfielder that;s consistent. That being said, even if he does that, IMO it still doesnt excuse 5 inconsistent seasons (does my memory serve me right though, i think Ole's first season, Pogba was quite good? so maybe 4 inconsistent seasons)
PS: you're right about the managers, Its only two.