Player Drop Off and Accessibility/Diffculty

[Absurdity]

Understandable, that's a fair opinion. However, the game has always been like this, and it used to be much worse, so I'm wondering why it you didn't feel it was a problem before?
Bosses often had obscure mechanics unique to them that you may not see again for the rest of the expansion, take Diabolos's portal doors from Amdapor, or the boss before having the tank kill an add to be able to draw aggro in the first place. These mechanics could get quite punishing, too.

I think the added variety of having to figure out what the boss's choice of communication will be is fun, personally. I also enjoyed ARR+ mech design best (or what was left of it when I began playing in 2020, many old dungeons would consistently wipe careless groups when this doesn't happen anymore sadly) where things are overall slower, but there's more moving pieces.

I can't tell you definitively, but I can give you my guess.

It's a problem now because people have gotten too comfortable with bosses just doing "dodge the same 5 orange AoE patterns while it casts for 10 seconds" for 3 expansions straight.
They even went back and reworked the mandatory ARR dungeons to do exactly the same thing, or they are so powercrept that you can simply ignore the mechanic.

So most players now have never even experienced having to figure out a boss' unique mechanics from whatever obscure tells it had.
Best example is Sunken Temple of Qarn. People regularly die to doom despite there being an obvious glowing platform and nothing else in the arena, because the game no longer bothers to make them develop the skill of "look around at what is happening".