Obviously he can still become a quality player, but we have enough of the hard working midfielders who are average on the ball. Herrera, schneiderlin, even schweinsteiger.... none of them are really great or creative on the ball, they are more players who add energy to our midfield.
Also besides, yes players can develop and change to an extent, but a players' individual play style is basically what it is and that wont change too much. If a player is an energetic midfielder who will get stuck in and drive forward with the ball, he is never going to be the play maker type, which is definitely what we lack in midfield. Someone who can pass the ball around properly. Thats why when carrick has a good game, everyone goes on about how much we miss him. He is the only one capable of hitting good passes around the pitch basically. Sanches, from admittedly the very limited time i have seen of him, is not a play maker type who has excellent passing range or a tight touch where he can dictate tempo through the middle and spray passes around and open the game up. Thiago, gundogan, pjanic, verratti, modric, rakitic... those are the type of players we need. Not another hard working player who is average on the ball. Especially not 60mil for an 18 year old. The price isnt even what bothers me that much, but spending that much on a player who doesnt fit the style we need is certainly a factor.