Manchester City until now played without De Bruyne and they can still achieve 3rd position in Premier League and won 2 matches in Champions League. They're already been great so far, even though De Bruyne is currently a Manchester City's heart.
De Bruyne will also get older, it's just a part of regeneration and Manchester City seems quite success since they're still in good form.
Yes, Manchester City is still performing well even without De Bruyne, thanks to other quality midfielders like Julian Alvarez and Phil Foden who are in excellent form. However, Manchester City is involved in several competitions, including the Community Shield, UEFA Super Cup, FA Cup, EFL Carabao Cup, Premier League, and the Champions League. So, De Bruyne's return is crucial for Manchester City's ambitions.
Overall, Man City has a relatively good performance even though they are playing without Kevin De Bruyne. But if we look at their last 4 matches, Man City seems to have experienced a decline in performance, and I think this is a problem that Guardiola must immediately address. Kevin De Bruyne is one of Man City's key players, and it is very likely that Man City will be a lot stronger when Kevin De Bruyne can play again.