I’m saying that the only mechanic that necessitates healers is: Unavoidable damage.
And the only mechanic that necessitates a tank is: A large amount of unavoidable damage at a single person.
Consider A11 where the currently closest target in range is being attacked by an autoattack that just barely does more than you can self-sustain. All the mechanics are still in place and still do lots of damage if not done correctly but zero otherwise. What does that lead to? The only method to beat him is to do the mechanics of the fight correctly (Hide, dodge, activate, burn etc). All the strategy involved is precisely what you currently are supposed to do anyway, except that the “tank” regularly needs to be swapped by another DPS stepping closer to him.
Is any teamwork lost in the process? I argue no – to me, teamwork is human to human coordination (Such as the “don’t get close” part of the judgement Nisi mechanic), not machine to human coordination (such as focus healing a tank buster/prey target). You don’t even need to forego things such as positionals or synergy effects via disembowel. Healers and tanks only add unavoidable damage to the boss mechanic roster. And that’s for a simple reason, too:
The only resource that matters is HP. The only way to reduce that resource is damage. The winning condition of a fight is to reduce the enemies HP before your own runs dry. Not just in this game – shooters, fighting games and many more work on the same principle. Healers add nothing to this system except increasing your own HP by their own mana pool times conversion rate. Tanks add nothing to this system except increasing the effective HP by reducing the incoming damage. Both exist solely to increase the HP pool the boss needs to deplete and in turn, they only add more damage to the boss to bring the equation back into balance. And unavoidable damage at that because otherwise good players would be inclined to just replace them with more DPS. That is all tanks and healers add to a game.
There also, naturally, is the satisfaction of different fantasies – some players just want to be sylphie, or that stalwart, impenetrable wall etc. But gameplay wise, healers and tanks add a lot less to a game than people give them credit for.