The original "holy trinity" term didn't refer to roles anyway, it referred to three specific classes in the original EverQuest - Warrior, Cleric, and Enchanter. The idea was, even by the time the first expansion came out, those classes were so good at what they did that you needed them to form any group, and other classes were just sort of...there. Remember that in the EQ days you needed a group to do anything at all.

The OP is somewhat right though, in that Warrior was a tank, Cleric was a healer, and Enchanter had the best crowd control. Again, because you needed a group for everything because even regular mobs were very difficult to grind for EXP, you also needed someone to perform crowd control functions in all of them. Of course there were more pure DD classes, but the "holy trinity" is what you needed to get things done.