The issue I have with people saying "Fergie made a mistake" is that I don't recall that being the general consensus at the time.
We all wanted him to stay and we could all see that he had great potential, however when he left we all said his heart clearly wasn't for staying (maybe influenced by his agent), Fergie knew best and good riddance. Now he's putting in some great performances two years later and people want to have a go at Fergie for it?
The manager made a decision based on what he knew at the time - in fact, maybe Fergie tried his hardest to keep Pogba but he simply couldn't persuade him. At the end of the day, we've missed out on a good player; it's happened in the past and we will do again in the future. It certainly wasn't Fergie's "fault" in my opinion.
Of course, we'll never know what truly went on behind the scenes. Purely from the perspective of him being a great player, then yes I would take him back. You have to imagine that were we to bring him back in the next few years, then Moyes would ask Fergie about the circumstances surrounding Pogba leaving and would use the advice to make a decision. If that decision was to bring him back, then I would support that. On the other hand, if it was decided that he wasn't worth the hassle, I'm not going to lose any sleep over it. Same principle applies to Ravel Morrison too.