It's shiny new toy syndrome.
No doubt he's a talented player but how high he's rated by some here is certainly baffling.
His best attribute is being able to dribble and carry the ball well, an attribute perfect for a counter-attacking side that exploits teams who commit. That and good passing ability, those are his standout qualities.
You then look at the current city team and you'll see those are qualities the likes of bernardo, mahrez, foden and even de-bruyne posses and i'd argue all are better than him at. Only difference is playing for this city side they don't get to showcase it much as positional discipline and systemic/patterns of play are more of a priority for a side that plays against sides packed in their area 99% of the time.
So he's not coming to help the team create any more chances than it already does with the players it has and the way it plays. You're then left with goals and it's almost laughable when his is compared to some of the even much maligned players here. His best goal-scoring season to date is 10 goals and unless some believe he has this unlocked potential to chip in with 15-20 goals a season as a winger it definitely hasn't been shown yet.
But somehow a player who's best with the ball at his feet but doesn't score much is suddenly going to improve a team with a bunch of players with similar attributes but also chip in with more goals. Honestly the only thing he could offer the team currently that it doesn't have is the ability to win cheap free-kicks, but even that may go down if he were to sign.
A top class striker should be and I'm sure is the main priority this summer, anything else like a grealish will just be a bonus to offer more depth and options to the squad but he certainly won't be this game-changing player to the side many think he will