My feeling that City may have reached their zenith is an opinion which is not wildly improbable.
Many United and Liverpool fans, and even some City fans, had the feeling that when Aguero, D. Silva, Kompany, Fernandinho...leave, City wouldn't be the same force any more. We had City fans predicting that we may struggle to make the top 4 in 20/21. After we drew with United and WBA in December 2020, many were pessimistic and thought Pep was finished. Guess what, City won the next 21 games and went from strength to strength, winning 3 league titles in a row and the Treble. The opposite happened: City got stronger after losing some legendary players, not weaker.
Now the narrative is: Gundogan left, Bernardo might leave, KDB is ageing, their dominance might be over. But Pep is quietly building the next team, with plenty of great players who are (not yet) in their prime: Ederson, Stones, Dias, Ake, Akanji, Gvardiol (?), Rodri, Grealish, Foden, Alvarez, Haaland... The likes of Haaland, Foden and Alvarez are likely to get better! The transition from a midfield with Gundo and Bernie to a midfield without them (and it's not even clear that Bernie will leave) is likely to be smooth, given who is the manager. The defence and the attack don't need big changes, the players are in or before their prime.