I don't think the answer is as simple as the excuse that the more modern generation of kids playing are worse than the people who were kids in vanilla/TBC. Honestly I think it might be a reflection of how raiding used to be. Bosses were really easy mechanical wise and sometimes you would bring obviously crap people just to fill the numbers. Now there's a lot more mechanics and research behind your class. Apart from the talents and stats balancing (which has always been a part) you had the legendaries in Legion and the azerite gear in BFA adding even another layer of "whats best for me?" and simulating. I think currently you need to sim yourself if you are somewhat serious because sometimes even a 415 trinket is a downgrade to a 400 one, same with the azerite pieces.
But I think LFR has played a big part in dumbing down the community as well. I don't take part in it myself and it's very easy to ignore if you play the game seriously, but I would imagine it's still the first "raid experience" for new players and I feel bad for them honestly.