I have some doubts about Man City after looking at their transfer market, a lot of key players have been let go like Mahrez recently joining the Saudi Pro League. For sure they need some time to get into top form after so many changes in their main squad.
I actually have no doubt about Manchester City. They might have lost a lot top players and will most likely lose more, but I know what Pep is capable of. Also, the transfer market is still open and there is still a lot of time for them to reinforce their squad. Mahrez and left and they're looking for a replacement for him. They are talking about getting Raphinha which would be a very good replacement. The good thing about Pep is that he can turn Raphinha to a world class player, the same way he'll turn Kovacic into a world class player.
I heard a swap deal with Barcelona for Raphinha and Benardo is on the table.
Pep is a genius. He needs good players to succeed quite alright, but the players in Manchester City is ready for anything. All the need is like three players; a right winger, a right back and a central midfielder and they will be good.
Keeping the same squad will not guarantee the success of the team another time, that's sure since all teams are changing players, coaches, management and tactics. The old players are getting older, there isn't a lot options for using them and giving some changes while making the team younger will guarantee success on the long run eventually. Guardiola know that and he is trying to keep keys players while giving younger ones a chance to play with them.