Well, in a sense the game already allows you to circumenvent the trinity-requirement: Level arcanist and you get both the DPS and the healer at 60, even if you only actually play SMN or scholar and just do the quests for the other one. So I dont see a real issue here to be honest - as you point out yourself: There are other requirements in the way of becoming a mentor (and they're more difficult to reach than three classes at 60)