This is so true. They in all likelihood haven't done high-end raiding in years if they ever have, if you could even reasonably compare some of the older content to modern Mythic. Encounter design has evolved so much, and on average encounters are in reality more difficult or complex, with the primary difference that organizational mechanics were more prominent (e.g. raid prep - resist gear, getting 25 or 40 people vs. 20 to show up at the same time, etc.) These organizational mechanics added some difficulty, but merely as barriers to actually playing the subgame of raiding.
Pruning the number of players down from 40 to 25 to a fixed 20 for the highest difficulty has only been a good thing - it's pretty much the optimal number of players to make the organizational mechanics have an impact, but not a major one, while allowing for specific encounter mechanics to be tuned to a sufficient difficulty. And Flex everything else is there for accessibility.