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.
And in my opinion, in the UCL group phase this season, Group F is the fiercest and up to now the four teams still have a chance to qualify from the group and even now it is still very difficult to predict who will be able to qualify, because these four teams have equal quality.
Group F is proving hard to predict. Despite teams like PSG and Dortmund not performing up to expectations, Newcastle is causing a surprise in this group. In my opinion, Newcastle and PSG will be the next two teams to advance.