No he didn't. Letting such huge talent walk on free was a mistake especially when you consider that our CM was screaming for talent. If Conte and Allegri could work with Pogba then so should Sir Alex. Sure Pogba was never going to retire with United. However we could have built him up and then we could have sold him off just as Juve did.
Pogba's limitations are mental. He simply lack the defensive awareness and discipline to play in a deeper role. That doesn't mean that he can't be brutally effective as shown both with Juventus and with United last year. You have to play him right.
I don't think SAF letting Pogba go was a mistake, never did. He didn't fit in with the culture of the club then. I admit I was optimistic when he returned, but what we have seen since shows exactly why letting him go was a good decision. He is talented, but inconsistent. He wants a team built around him, but he's not ready to lead. He is more likely to butt heads with his manager than accept a bollocking and understand that he needs to improve. He's also popular with the players, which makes the above a recipe for dressing room division.
Conte and Allegri being able to work with him is good, but I still rate SAF opinion over theirs, they are good coaches, he is an all time great, especially with regards to man management. Sometimes players can't be helped if they don't have the right mentality and desire. Ravel Morrison, Pogba's team mate and arguably the more talented of the two, is a more extreme example.
I think what we've seen is Pogba's level, talented, but inconsistent, unmotivated, tactically unaware with questions over his professionalism. I don't think he really wants to improve. If he moves to a club where he is surrounded with talent or to a less demanding league his weaknesses may not be so obvious, and he may look as good as he did at Juve.