Hmm in the case of De Bruyne, I wonder what will happen to Manchester city when this guy will finally retire as they are yet to get his perfect replacement
They have no choice other than get a new player that can replace him. It doesn't mean manchester city needs to get someone who will be replicating de bruyne. The club is currently seeking a player who can perform at a close distance to him, and finding someone who meets that standard would be satisfactory. I recall that there have been discussions about Dani Olmo being a potential candidate to succeed De Bruyne, however, it is important to mention that Pep's latest statement hinted at his willingness to see De Bruyne retire at Manchester City.
Yes, actually there is no one club that carries out major changes or is willing to let go of old players and replace them with young players to maintain performance in the long term, so if one day Manchester City management sells Kevin De Bruyne, I will not regret it as long as I can get more talented young players or don't have to have the same performance as De Bruyne but at least have quality talent.
Regarding Dani Olmo, I'm not sure he will be chosen by Pep Guardiola as a replacement for De Bruyne because Dani Olmo doesn't actually have that good talent, it's just that he's still a little younger and that seems to be just a rumor.