And I am his prophet.
What is it that keeps the idea of a tank alive? Yes, exactly the same thing that keeps the tank alive: healers. Without a healer, even a tank will die. And when the tank can die, is he really a tank or just awful DPS who survive a few more seconds but do a lot less damage? Yes.
So I introduce to you a magical world in which queues are instant, no one heals, and no one tanks. In this magical world everything hits a lot softer but has a lot more health. Rather than healing or tanking, we just take turns casting Taunt Everything and then bandaging.
Does this sound skilless and boring? Of course it does! I bet some players might even enjoy it. So of course we have to fix that.
Step one: Add a few mobs with AoE. These will interrupt bandaging, ruining the taunting rotation since the next guy in line will still be half-dead. So we need to focus on them, possibly with death or possibly with CC.
Step two: Uh.
Okay I give up, I hate this idea. We're all doomed. If we separate out roles, we're going to have shortages which vary wildly with the content. Heroics: not enough tanks or healers. Raids: too many tanks, not enough healers. Great system! If we remove the trinity then it turns into a muddled pointless mess of taunt rotations or even worse, we have to all play dynamically with cooldown usage, kiting, and CC to prevent death.