For the Trinity to work, each role needs to require both of the other roles to be present to clear content.

As it stands right now, tanks do not need the other roles to be present in Normal content. They have sufficient self-healing and mitigation so they don't need healers unless there's a Doom to be cleansed. They don't need DPS because normal content rarely includes a tight DPS check outside of the 8 man trials (and you can probably clear those with 8 tanks).

Healers only need one of the two other roles present. Tanks to hold enmity so they can focus on damage (weak as it is) or DPS to hold enmity so they can focus on healing.

DPS only need one of the other two roles present. Like healers, they need tanks to hold enmity so they can damage. Or they need healers to keep them alive as they both hold enmity and damage. If the vast majority of damage in an encounter is avoidable, they don't need either.

Tank design is the major broken link because it so rarely needs either of the other roles outside of high-end duties. The game is at the point where everyone might as well play a tank because there is so very little that a tank can't handle outside of Doom.

It's on SE to decide they want to bring the Trinity back into "balance" by making each role require the other two in group content.

Quote Originally Posted by Valkyrie_Lenneth View Post
Without the trinity fights would have to change. Either every job can take any hit from the boss, or all big hits are avoidable. Tankbuster could no longer be a thing.
DPS are already able to take tank busters in just about any content outside of EX/Savage/Ultimate.

Give their gear the Defense and Magic Defense Ratings on tank gear and an alert healer, and they could probably take some of the higher end tank busters that don't require an invuln as well.