Firstly, the point is to have a forward or winger who can beat a man and create chances. We have plenty of left sided inverted forwards. If all the players move in field from the left side and the right, the space they would play in would be mininized. We have already suffered from that last season where the distances between players have looked completely wrong and team has looked disbalanced.
The width maintained by the forward or winger is maintained to provide space for other attacking players through the middle, like Sane or Ribery and even Neymar, when they hug the touchline, provide width and strech defences and create spaces through the middle. An inverted winger doesn't imply that the winger would always move in field to make space for the fullback. That would make everything so predictable. An inverted winger offers a dual threat of going in or out. And the ones that can do that, have genuine pace, can beat defenders consistently and provide end product are all high valued players.
Let's get real, under Jose we are not a team that allows both its fullbacks to join the attack in tandem often enough esp. against top teams. Moreover, we already have an inverted forwards on the left in Sanchez. It would provide balance to the side to have a world class winger on the right who can create chances for goalscorers like Sanchez and Lukaku rather than someone who can only be at the end of those chances.