I disagree with most of the reasoning against Grealish working for City. I think he's very talented, I think he works hard, I believe that in a football capacity he has a good attitude and work ethic, I think he has the capacity to improve under a top coach etc, I think he's happy to work for the team etc etc. Given his interviews I think he'd love to play for City too. He'd really excel.
However, I still wouldn't want to sign him because we simply don't need a player of his position/role. With De Bruyne/Gundogan/Foden/Bernardo (not to mention a possible incoming Messi) it leaves us far too overcrowded in that area if he signs. A Grealish type player is as necessary as us signing another CB ie it's not.
Maybe things could change in the next couple of years and we'll need a guy like Grealish in which case I'd welcome him with open arms, but right now, no we shouldn't sign him in my opinion.