@ Kish: I hear you. Hopefully enough of the classic server crowd want your specific version that that is what you get to play.
Well you could ask the exact same bolded question of games like Destiny and Warframe (and Anthem and Firefall and Em8er and...), and my answer would be the same to you there - the MMO format provides long-term progression/rewards, an evolving world/story - and yes, community (I'm not opposed to it, I just think it should be more tightly regulated than it is now.)
And I think I've said this to you multiple times in the past (including in other game threads - ohai, Overwatch!) but I just don't have the experiences with rampant "pubtards" that you seem to. But even if I did, my recommendation to the designers would be the same - give us tools to coordinate our assault that don't simply hand a megaphone to any jerk who wants to be toxic.
Oh, I know material rarity would still exist, I just don't think it's enough outside of Warframe's model. I could be wrong though, given that I have a sample size of one successful game that uses it. If there's a subscription-based game that relies primarily on crafted gear rather than drops, I'd love to see how they do it.
You're right that L4D's AI Director works well, but you're forgetting why that is. Remember, these are two completely different genres. WoW, like most RPGs, is meant to be empowering - the challenge should feel tough enough to be engaging but ultimately, the fun is in winning and feeling like a hero/badass. Left 4 Dead meanwhile is intended to be disempowering; your goal is mere survival and lurching from safe house to safe house by the skin of your teeth. Thus when the AI makes things easy in L4D, rather than getting bored, the players feel tense - it's the calm before the storm, and it means a tank or witch or boomer is probably right around the corner, thus they stay engaged. And when the AI makes things crushingly difficult, your job is to get everyone who is currently disabled by a superzombie on their feet (or leave them to die!) and haul ass to the next checkpoint. Lulls in a WoW instance meanwhile simply mean you have more of your cooldowns ready to go, not to mention being able to take a drink to refill your resource bar (for the classes that even need one) too. In short, L4D has a much, much wider band of difficulty to play with (both the peaks and the valleys) - and what creates valuable tension in one game, does nothing but destroy it in the other. Oh yeah, and lest we forget, all the L4D characters are identical in capability, while WoW classes have to play and feel different from one another.
Could they make some similar kind of automatic-difficulty-adjusting algorithm for WoW? Probably, but the level of tuning it would require to keep the peaks and valleys from getting too far out of whack (especially across every class and spec) would be nightmarish. Which is why Diablo and Mythic+ just hand it to the players (before they start the run.)