He won't, that is a myth. Most of his value today is in his age, potential and hype. He'll only get older and unless he has an absolute monster season were he's head and shoulders above every midfielder out there his price will drop.
Just look at Vidal, he's a better player than Pogba and has outperformed him yet there is no way we'd get 70m from a team for him and that is because Vidal is 26 and not 20 like Pogba.