I do think we will have a slightly better team even if Pogba leaves, but i don't think we made this massive jump forwards that some think we did. Morata always was a force in important CL games, it's tough to imagine that Higuain can top that for example.
Keeping Pogba is the missing piece of having a team that is better than any team that isn't Barca, Real or Bayern. Without him, without a top-class midfield, that possibility vanishes. Matic or Witsel instead of Pogba won't take the midfield to the next level, while signing Pjanic, while simultaneously keeping Pogba would.