I don't care about what is the case "At the higher levels". I care about what is the case in the game. There is no reason to limit the discussion to the top fraction of a fraction of a fraction of a fraction of a fraction of players.
The availability of players who want to play a role is directly caused by what playing the role looks like.And if they're roughly the same at the higher levels of play then I can't imagine much is different the whole way down. I say that as somebody who has played all three roles in M+ above +20s. (I don't raid anymore but I was a CE raider from the end of WotLK through MoP and I played all three roles then as well.) The problem isn't the difficulty of the specs or roles, imo, but rather the availability of players who want to play them.
The idea that all Blizzard, the creators of the game who can make almost literally any change they want, can do is tweak affixes around the edges is so silly that I don't really know how to deal with it. They could nuke those affixes. They could change the entire affix system. All of this is artifice. All of it is changeable.Some affixes also make tanking or healing more of a chore than needed. (Necrotic, for example, is exceptionally punitive and makes finding tanks an even more arduous process than it is normally. Explosive is also seen as a "healer mechanic" so healers are more difficult to come by on these weeks.) Blizzard can do more to incentivize players to run keys on these weeks with things like reward boxes or extra gear but ultimately that's all they can do.