You know, we actually had a point in this game's lifetime where the trinity functioned in dungeons, it was Stormblood.

If the tank was bad, aggro goes everywhere but a good healer can salvage the situation, DPS can also help peel for the healer by juggling mob aggro.
If the healer was bad, tanks couldn't sit in damage stance but could salvage the situation by using their defensive side of the kit, DPS can also help by burning down targets one-by-one to reduce pressure on the tank.
If the DPS were bad, the tank and healer can still ration out their resources to salvage the situation.

It was a pretty decent trinity game back then because every role had ways to take control of a situation that went bad while still needing to rely on each other. Tanks also weren't as squishy as people made them out to be, I soloed a few bosses on tank back in SB, the difference back then is that we could only do it from 20% HP max, nobody could solo a boss down from 90% no matter how good they were back then.