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 Perhaps Pep's era is coming to an end, perhaps his strategy is outdated, or his strategy is easily read by opponents, or perhaps his strategy has no further improvement which is making Man City doubtful this season.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 Guardiola's tactics have become outdated.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.
When looking at successful managers in the past, there's always a downturn, and this is what's happening to Pep Guardiola. It could be said that every cycle has an end this could be Guardiola's last season at Man City if he fails to win any trophies.
The signs are already visible. They've been in the Premier League for three weeks, but Man City have lost twice in a row and won just once. I'd say that Man City isn't at the level they'd expect right now because they easily lose focus and make big mistakes. Furthermore, Guardiola often makes blunders in signing and selling players. He often recruits players who don't fit the league his team plays in, especially since Pep hasn't found a creative player like De Bruyne. I also doubt Man City's strength this season.