I think this is not entirely Pep's fault before he decided to recruit players, he must have discussed it first with the club management, not just carelessly recruiting the players he needed, especially Man City last season was hit by many accusations so they have to be careful in acting if they don't want to make a fatal mistake, bad luck is indeed no one knows Pep Guardiola must feel quite confident with the existing squad because he thinks he has mastered the game for years but it seems this season he has run out of tactics and his strategy is starting to be easily read by his opponents.
If for that reason Manchester City is experiencing a decline then I don't think that is an excuse because the squad that Pep Guardiola currently has is not much different from last season's squad and it shouldn't affect the team's performance that much. Maybe because other team coaches already understand Pep Guardiola's strategy so they can see weaknesses that can be exploited and are also influenced by several players who were previously injured.
It feels difficult to maintain dominance in the English league even though Manchester City has done that for several seasons in a row. Now there are much better teams then Manchester City will experience a bit of a setback and it is only right for other clubs to try to dominate so the league will be more interesting and not boring.