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.
It is very difficult to maintain a high level of play over several seasons, after every peak there is a decline, and besides, other teams can be much stronger next season, until we know how Bayern will play, or what to expect from Real Madrid, Barcelona or other teams . Guardiola did everything right and City looked very good at the end of the season, but in previous seasons everything was not so perfect, so I'm not sure that things will turn out as well for City as they did this time.