Replacing De Bruyne is not a given, he is an exceptional player, who in my opinion could still give a lot to Manchester City, but they decided to give him to Napoli who signed an exceptional player, regarding City it is difficult to find a player with those characteristics, but it must be said that they did not do much to find him
Finding a player who can replace De Bruyne will indeed be very difficult. De Bruyne's quality is already at a fairly high level. Therefore, even young players will need time to reach the same level of maturity as De Bruyne. De Bruyne is still good enough to be retained by Manchester City. However, Manchester City doesn't want to take the risk. We know De Bruyne is a player who is quite susceptible to injury. He has suffered quite a few injuries in previous seasons, and Manchester City seems unwilling to take that risk. Therefore, Manchester City decided not to renew his contract. However, Napoli were truly fortunate to get De Bruyne for free. Meanwhile, I think Manchester City were unlucky for not taking the risk of extending De Bruyne's contract, which was actually still needed.