I think things may not be as the same for Manchester City in next season because other teams are trying much as possible best to strengthen their team because they already know how tough Manchester City is. Teams like Chelsea, Manchester United and Arsenal won't be easy with Manchester City next season, so if these things are trying to bring the best squad I don't think it will be easy for Manchester City to have that strength to defeat this strong squad coming up in these teams.
City could not keep Gundogan in the team, this is perhaps a very significant loss for Guardiola, even a salary increase did not help, but Gundogan was more interested in the duration of the contract and Barca offered him a contract for two years. It's hard to say how significant Gundogan's absence will be for City, but he was without a doubt one of the most important players in Guardiola's scheme. But there are no irreplaceable people, so Guariliola will find a replacement for Gundogan.
Gundogan departure to Barcelona wouldn't be a serious issue for Guordiola he is one of the best manager in the world thus he can easily replace Gundogan role in the team, I think Gundogan personal desire to play for Barcelona when the opportunity came up was responsible for his departure thus issue of contract improvement isn't the problem, however recall when Man City was having issue with left backs in the squad yet he allowed Joan Cancelo to join Bayern Munich on a loan deal despite that Guordiola still won the treble, issue of key players leaving a strong club and with a perfect replacement of such players is one of the yardstick in rating a very good manager thus the team will still remain formidable next season.