Kevin De Bruyn barely played in played regular football in the last couple of season at Manchester City yet, in the season before the last one, they won the English Premier League. I think there is more to Manchester City's woes than just players or maybe the oppositions has got harder, and they are now well versed with Guardiola Ball.
Remember how Klopp's Liverpool performed vs When Arne Slot took over? He barely signed anyone, and yet they were so impressive. Sometimes, all it takes to change is just the coach.
I agree with your points about team performance. Since Manchester City won league even with Kevin De Bruyne missing many games their recent problems are likely more than just single player absence. It is possible that other teams have now learned how to play against Pep Guardiola style. Your comparison to Liverpool is perfect example of how new coachn can make huge difference with bringing new tactics and energy to team without needing to change players. This shows that sometimes coach is most important part of team success.