There are so many facets to this. Let's start with the obvious ones: Sports science is improving. Performance diagnistics, nutrition, medication, recovery, injury prevention, the cognitive part of how to transport your idea into the heads of the players (through modern training exercises (there was a really interesting article how Tuchel had cut games completely from his training routines), but laso technological advances and visualization tools), etc. Very important also: You have much more tools, at times even AI preparing analyses, advanced metrics, visualization tools, and so forth. Plus the increased budgets of football clubs means they have much more resources at their disposal - complete scouting teams that take care of the tactical analyses. Somebody identifies a tactical weakness in the upcoming opponent's line up and the coaching team knows what to train to exploit it. Barca and Liverpool for instance have some of the largest football innovation hubs in the world, working together with dozens of startups. A few years ago I read how Valverde's team prepared the players on their next opponent's, e. g. handing out Messi detailed scouting reports which players have which habits and where spaces in the opponent's defensive line up tend to emerge, etc. And also the availbility of data you need to scout and improve your players: There are so many providers of sports data these days that are used by professional football clubs.
In the end, football clubs are businesses and businesses are always looking at how to improve their processes, tools, productivity, etc.
To be honest, I don't really know what you're referring to? I brought up many arguments, even factual ones.
Of course it does! But that's part of the equation, you can't just ignore it. We're talking about the end result here (which team/period was more dominant/better), not how you got there or how impressive that was (I'm not arguing that what SAF did with United isn't more impressive than what Guardiola did with City, that's an entirely different debate).
As said: The question is why did they fall short? And were they really that much worse or were they simply outpaced by certain contemporaries? I'd also remove Bayern and Atletico from that list. But what you're ignoring is that there have been new clubs that emerged: City and PSG at the very elite level first and foremost but also Liverpool (who reached 3 CL finals in 5 years).
I mean, even assuming that this is true, what makes you believe that we currently have a weak pool of elite teams? The concentration of top players at top clubs has never been higher, the financial gap to the runner ups has never been bigger. There's really not much speaking for your argument.