This is a cyclic argument imo. It's obvious that as time passes and opponents get to play you again and again and again they will get to practice more and more and will get better at blocking you. I'm not disputing that. It's obvious some evolution happens on the side of the opponent. But it's not the main reason why great teams (wouldn't call City a great team but still) decline. The main reason why they decline is that they just can't keep up doing the same routine at the highest level. That stress and that peak can only be handled for 3-5 years.
If 2009 or 2011 Barca would play today, a fresh team, they would dominate any team and would be favorite for every trophy. After 3-4 years they would again start to lose steam and would be losing more and more to sides they would have beaten in the past.
Especially 2011, they would dominate like the old times, basically unbeatable, park the bus and prey you score a setpiece, it was all you really could do.
Though, CL is CL, you can have the best team in the world by a distance, but silly/crazy things just happens in that tournament.