This is simply a matter of era, as you said. Perhaps Pep's era is coming to an end, perhaps his strategy is outdated, or his strategy is easily read by opponents, or perhaps his strategy has no further improvement which is making Man City doubtful this season. Why can we say this is a matter of era? During Pep's glory days, even fielding a reserve lineup could win easily. But when the glory days begin to fade, even with the best squad Pep will struggle to win.
I think that is what has happened. Guardiola's tactics have become outdated. The opponents are now able to easily understand Guardiola's tactics and because of this, Manchester City has to fight hard to win almost every match. Anyway , Manchester City's overall performance is now weak. There have been several changes in the squad. So the squad seems to be a little weaker this season than in the last few years. Manchester city can not find a worthy replacement for an experienced midfielder like de bruyne. The void created by De bruyne's absence from the squad is quite visible.