Even if the next tier is all physical, I can't see that sidelining DRK the way PLD is sidelined now, as PLD doesn't really have a physical equivalent of Dark Mind. DRK and PLD have pretty similar cooldown kits outside of that one ability. PLD does have Shelltron but I don't think that would be enough to really make DRK unwanted because they also bring the INT down debuff and solid DPS.