Not that I believe the deal will happen but hypothetically speaking £85m for a 28 y/o koulibaly is 10x a better deal than £75m for a 26 y/o maguire. I feel like some don't realise how good he is. He has literally everything you would want in a defender for this current city side. Physically strong, very good reading of the game, strong in the tackle and in the air, deceptively quick and very good passer on the ball with both feet. Imo there's almost no downside to this deal if it were to happen(him settling in the league probably being the only one) and he would easily be the best defender in the league(yes even over van dijk) that's how highly I rate him. You get a player for his peak years and at least 4 top seasons out of him if not more.
In fact the only deal where I'd value slightly more for a defender right now would be de ligt and that's purely for his age and upside but if you were to ask me which defender out of de ligt, maguire and koulibaly for the here and now who could elevate it than it already is, it would be the latter without a doubt. There's enough 'young' defenders in the side as it is, what is needed now especially with kompany gone is the experience and know how that he would bring which proved crucial during the final stretch last season where I actually felt he bailed and cleaned up for laporte a lot during that period as mistakes started creeping in his game and as good as he(laporte) and the others are none are on koulibaly's level yet who is currently what kompany was at his peak.