No you thought shit longer than me mate. The amount of bs in your post is straight up staggering. It doesn't matter if raid bosses won't be designed around covenant abilities, everyone who raids mythic (with a few exceptions), needs the best setup possible. Why?
Because top 100-200 raiders will strive to do the best they can stay competitive and get as early rankings as possible. It's not about the boss being impossible, it's about the fight becoming infinitely easier sometimes with the right setup. Did we need 4 warlocks on G'huun mythic? No we didn't when we killed him with my guild, did it make it infinitely easier with 4 portals instead of 3? Even though our 4rth warlock was shit dps wise? Damn right it did. Did it earn him a spot over our ret paladin who had nothing to offer for the fight? Absolutely.
If you fail to understand this, you are a small boy, who is trying to tell everyone to git gud, but lacks the mental capacity to understand how the damn community works. And for guilds who barely get cutting edge, the optimal classes/builds/etc, are a way to help them achieve eventually the kills. No sane man will try to progress cutting edge and not go for optimal build. I think it's you who needs to git gud because it seems like you've never even played the damn game.
Also your 'reverse question' is subject to the same flaw - If you have 21 raiders, 10 of which are key members (like 4 healers, 2 tanks, and 4 dps who are super good), and then you have ELEVEN more raiders, who all play immunity classes aside from that one enhancement shaman who is an extremely good player (always parses 90-95+ on mythic), is always there, consistent with mechanics, etc, and the 10 ppl who play immunity classes are all extremely good also, but play classes that are far stronger (fire mages, bm hunters) and can do much more dmg, while soaking with an immunity. What do you do then? Do you bring the shaman? Why? The immunities will make the fight a lot easier. The shaman is losing out on progression.
In shadowlands, this will happen with covenant abilities sometimes (like the venthyr teleport), and why the fuck something that should be an aesthetic choice for the most part, and something that was made to be fun, has to put you in such a spot? If you fail to understand all this, we are done discussing I think.