Manchester city should not fight the situation, they should take out time and resolve it, this same players your admonishing are the golden boys of Manchester city, they've been defending the league title back to back for four seasons now, and it is natural for there to be redundancy in their performance since they are humans and not machine's, fatigue and injuries are the major factors that usually affect a top club that performs at optimum level like Manchester city, if they try to come back fully into dominance and at the end of the day, they could not, the club management board should take a chill pill and not rush into any action that might be detrimental to the team by signing players they might not be needing, if they lose out this season on the title race, then next season is their for them to pick up the pace again.
First of all, it's true that most of their players are getting older and they need younger players, but also I would like to point out that they are doing a bit of rebuilding while also keeping their old players as well. Gvardiol was a great transfer, only 22 years old (even though he looks 40 lol), Doku is 22 years old too, and Foden isn't a transfer, but academy player, only 24 years old, and obviously Haaland, he is 24 years old as well.
So, we have 4 players already that are under 25 years old, and if they can continue to rebuild with like one or two younger players every season, they should be fine. But that's a bit hard, because what 23 year old could play as well as KDB? Or Silva? Or most other players, it's a tough deal, they would obviously love to rebuild, but it's difficult.