I think the reason why Manchester city performance declined this season is because of some injuries their players were experiencing, especially their key players, De Bruyne and Rodri. If these two key players had been fit throughout the season, Manchester City form would have been better than how they performed this season, and they may have likely won the Premier League again.
I know that injury really affected Manchester city, and that’s the cause of the decline in their performance, a club that won the premier league for 3 seasons, then the same club was struggling to qualify for the champions league. De Bruyne and Rodri absence really affected Manchester city last season, but now that Manchester city didn’t renew De Bruyne’s contract, I think his absence is going to affect Manchester City next season also. Even as we can see that he is old already, he still has great impact in Manchester city’s performance.