Pogba would definitely leave united, I doubt if he is happy playing for united anymore, going back to juve yet again isn't bad and it'll be a great addition, but I doubt if Pogba has what it takes to win the ballon d'or.
The Juventus Pogba got everything needed for aiming to that.
2014 22° Paul Pogba 22yo
2015 15° Paul Pogba 23yo
He went to ManU shortly after and lost his way. (According to Wikipedia)
Pogba was amazing with Juventus. That was a reason that he was the most paid player in the world.