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.
It was wonderful that Manchester City were able to play and win without De Bruyne; they should get used to it now, as should other key players in the club, so that when they are out injured or suspended, they can still play and win all of their matches till they return. If it wasn't for Rodri's suspension, which resulted in two losses for Manchester City, I don't think De Bruyne's absence would have had much of an impact. Manchester City will undoubtedly finish higher than the other top clubs in the league and in the Champions League games.
Yes, maybe you are right. 4 more matches will be played. As you said, Manchester United will win these 4 matches and Manchester United will finish on 1st position. But right now, as I said, things are not going well for Manchester United. Casemiro is suspended due to a red card. There are many injured players. Also Manchester United's performance is very unstable.
Even before several of their players were injured and underperforming, Manchester United's previous performance cannot be compared to that of Bayern Munich, and they are far superior to them and will finish ahead of them in the group stage. Manchester United still has a chance to finish second in the group because they have four more matches to play, but I doubt they can beat Bayern Munich in their second leg clash. If they lose or draw a point with Bayern Munich, they will be disqualified from finishing first, and Bayern Munich will almost certainly win their remaining four matches.