With Silva rumored to be leaving last season, and Gundogan on the wrong side of 30, it was a purchase that made sense. If City had their way they would have got Grealish and Kane. Cancelo and Zinchenko play at LB, and Torres was signed as a wide forward. So your first sentence doesn't really make any sense.
De Bruyne's competition is B Silva/Gundogan, not Grealish. When he gets back to full form he'll most likely play as the false 9, unless his competition is injured or suffers a loss in form. Then the left wing option will be between Grealish and Foden.
Kevin Love/Chris Bosh?
But the comparison between basketball and football is somewhat flawed when applicable to Grealish. His "problem" isn't having to deal with not being the main man, it's learning how to marry his strengths with the way City play (one touch football, give and go).
He is playing on the left wing right now because Gundogan and B. Silva are great and understand the system better, also because mistakes made on the left wing are less prone to being punished. As a winger his dribbling isn't threatening. When he moves into midfield it'll become invaluable, as he has the ability to retain possession until the right moment. That was something David Silva excelled at.
There's no guarantee of this signing being a success. But the last few pages read so much like Caf commentary on B. Silva, Cancelo, Rodri, in their first seasons. If I was a betting man...