Of course there is no guarantee that Manchester City will be able to maintain what they have achieved this season, because other teams will surely improve themselves even better to navigate the new season to come. On the other hand, even though Manchester City is in very prime condition this season, it is not certain that they will be able to maintain it, because the decline in the team's performance will be unpredictable, we can see from what Liverpool experienced.
Liverpool performance were declining because Mane is leaving and several players were injured, at least if Liverpool players weren't injured, I think it's possible for them to achieve top 4 and qualify for champions league.
Manchester City on other hands, I don't think Pep will let their players to leave the club, so I don't think the next season Manchester City will play bad. Remember, this is the beginning journey of Haaland in Manchester City.