What gives you the impression that Manchester City won't be able to accomplish what Real Madrid did by winning the champions league back-to-back by the next season? Manchester City currently possesses the best team that can defeat any opponent. The only thing I am aware of that would prevent Manchester City from winning the Champions League by the next season is if they sold or loaned some of its important players to other teams, or even if some of their players sustained major injuries that would have a negative impact on the team's performance. Apart from that, I don't think anything can stop Manchester City from reclaiming the champions league title.
There is still an exception to this; they did incredibly well this season, earning them the champions league trophy, and no team presently holds their record in football history as far as the previously concluded season is concerned. But we can't be certain of what other teams' performance will be like next season in comparison to Manchester City's. While speculating, we may give Manchester City full confidence in winning the trophy again next season, but in reality, we need to observe how other clubs perform when the next season begins to determine who will win the title in the end.