First of all, if they get Pogba he will improve their team. No doubt about that. Doesn't mean the win the title or finish top 4, but Pogba will improve them.
Secondly, I find it hard to see the difference with them splashing out 100mill. on one player with a huge salary, while we will spent the same amount and a double salary on Sane and Stones.
Thirdly, if Pogba turns out to be Ronaldo 2.0 and plays for them for the next 5-6 years, Pogba will be a bargain. He will certainly be a selling shirt kind of player, probably beating any of our players.
If United doesn't get in to the CL next season (or Arsenal, Chelsea and Liverpool for that matter) it will hurt them as hell. If we don't it'll hurt us in the same way. Leicester, Spurs and West Ham to name three outsiders for the title, won't put "guaranteed CL" in their books for next year.