Although Pogba is certainly a good player, it must be said that the €105 million Manchester transferred to Juventus five years ago never paid off. Pogba is always good for individual situations, but on the whole he doesn't strengthen the team enough to play at the top nationally or internationally. It's better for ManU if he leaves the club and frees up money for better players.
Pogba didn't really become a star at United, but I wouldn't say he didn't recoup the amount that was paid for him. He showed a consistently good game throughout all seasons and his contribution was always good, perhaps if he scored more goals then there would be less talk about his usefulness, since such an indicator is easier to see and evaluate than the number and percentage of accurate passes. In any case, if he does not want to continue to play for United, then the club will have a difficult task to find a replacement for him.