It was a trinity game. It had tanks, healers and DPS. Plus controllers. Well ok, trinity-point-five.
Content could be completed with any group composition but as a general (i.e. not absolute) rule, 1 or 2 tanks, at least one healer and a bunch of DPS was the most effective.
Though archetypes and their power sets are more varied than other typical RPGs. An emp defender / controller is a very different beast to a kinetics one. Ditto the control primaries are varied. A team full of fire / kin controllers would instantly melt everything. Not so much a team full of mind / emps.
Personally I think they struck a good balance. It was an extremely rare occurrence to end up in a full team with a significantly sub-optimal powerset composition.
Have you been paying attention?
Guild Wars 2 at launch made much of their having done away with trinity. And they did. At launch, healers weren't really a thing in their game. They favoured damage avoidance and self-heals on big cooldowns.
End-game PvE was a melee zerg-fest. Yawn. Accordingly a lot of people played and enjoyed the game until max level, then got bored and left. Later they realised their mistake and tried to falsely claim that they never said they did away with trinity, or did, and reintroduced it.
Unfortunately - especially for us tanks and healers - many other developers seeing GW2's great financial success, mistakenly attributed it to this design element and also effectively did away with trinity.
eg. Elder Scrolls Online claims to have tanks, healers and DPS, but in terms of actual group gameplay, healers primarily DPS and mindlessly re-apply crazily short duration buffs and heal over times. Oh and never resurrect. DPS do that. Healers don't just in case someone actually needs a heal.
Healers were always my fave in online games and they've largely vanished in the past decade.