the need for 3rd tank class

[Assirra]

GW2 problem isn't that you don't need a "tank/healer" but that the content is piss poor of a design.
End game content in GW2 is mostly zerg this, zerg that and there is little room for strategy.
There is a couple of things that usually require some cooperation/orientation and those are actually kinda fun but also made absurdly difficult with little reward in the end (I am talking about some of the Fractals).

So how can you make challenging strategic combat without the holy trinity exactly? You can't let the boss hit hard because there won't be dedicated healers to heal or a tank to take the blunt of the damage. At best you can do some "interupt boss" or "move boss there"
There is a reason this combo got used since the dawn of roleplaying games.