He gives City a great plan B when tiki taka isn't working. Tactically it'll be a huge step backwards if they let him go and fail to replace his directness with the ball.
There's no disputing that he's an excellent option to have, but not sure that tactically he'd be a huge loss in the manner that you suggest. Bernardo Silva and Sterling are very direct players too. They both are aggressive in possession and can be relied upon to pressure their fullback and create one v one situations.