That's bullshit though.
Every boss requires the Holy Trinity.
The Trinity is only one possibility, though. I think I need to restate every month or so what you need for an interesting combat vs non-soloable mobs:
- The combat needs highly distinct roles by distinct characters.
- Those roles should be complementary: a role by one character should enable/enhance the efficacy of another.
- Those roles should be mostly necessary in a vast majority of fights; a fight that does not need one of the roles should be the exception, not the rule.
- Those roles should be obvious to the players.
The Trinity is one combination of roles that fills those criteria, but you can have other combos. A tankless combat system would possibly involve a swapper (who swaps unused resources from a character to another, like draining mana from a melee to give to caster, or endurance from caster to melee), a debuffer (who keeps the mob abilities suppressed) and a CC (because mobs usually occur in group, but your debuffer will be unable to keep debuffs on a majority of them).
It plays very differently, because a swapper isn't a healer: if you fail at your job, the group is less efficient, and possibly exhausted before the end of combat (which leads to wipe), and you don't have hard failures ("I couldn't keep the tank healed"), the debuffer isn't a tank (the abilities still hit whatever the mob wants to hit, but you counter them/reduce them), and the CC doesn't affect the fight, he merely makes it possible. But it would be no less interesting (and I'm pretty sure people would insist that the swapper is a healer and the debuffer a tank, even if the debuffer isn't hit even once in a fight and the healer never touches his group's HP).
But if you want groups not to require those 4 characteristics (with assertions of "you don't need a given role" or "all players will need to be ready to fill any role at any time during a fight"), the net result is not an interesting combat, because players will fall back to the default of "no role, no synergy, no cooperation", and your game will have to be nerfed to that level or 90% of the players will quit within hours of encountering group-based fights.