Even then, this is exactly like how FFXI played out:
Original Tank = Warrior
Paladin gets some fixes = Backup tank
Fastfoward through Zilart and many more adjustments
Warrior = more focused towards DD
Paladin = more focused towards tanking, becoming the key tank
Ninja = Sup?
Fastfoward through even more adjustments and level cap increases including more job and spell additions and ability additions throughout the years
Warrior = Pure DD and stagger whore.
Paladin = The tank. Period. (You can't argue against a very decent rate of restoring your hp with damage dealt, i.e mob does a 2.2k dmg WS you'll restore 2.2k HP.)
Ninja = Up and down in terms of tanking role.
So for now there's two clear tanks, but with further adjustments this can and will change, so it's hard to say MRD will be the main tank of the game when this is only 1 patch.