What the fuck are you talking about lol.
The warrior main tanks as he can take the most punishment and has great self heals that allow the healers to breath a little from time to time, the Dark Knight does comprable damage to a DPS and has a liteny of great options for both raid-wide and personal damage and swaps for mechanics that need it.
You can't get more off tank than that.
But bosses tend to enforce tank swaps so you can't just do that via debuffs. For most of this game's existence you didn't just optimally go "Warrior tank the entire boss" not even in Heavensward did you do that when Warrior was broken and by far the best tank at surviving anything. This is because swapping the tanks to kitchen sink your CDs into tank busters was always better then having one tank do everything.
Ideally, assuming we aren't forced by the boss to linger on one tank due to debuff stacking, you'd just swap tanks back and forth depending on who has what CDs available so the most usage of your CDs possible is used as having Rampart sit there doing nothing for 3 minutes is a waste of Rampart. That is the best way to use both your tanks, "off tank" tends to conflate with a bunch of pointless bullshit assumptions like being "the add tank" or "just dps until the boss forces the main tank to swap with you" which is not what you should be doing as that means your tank CDs just do fucking nothing for half the fight which is wasteful. Ideally both tanks tank the boss about evenly.
Beyond that this:
the Dark Knight does comprable damage to a DPS and has a liteny of great options for both raid-wide and personal damage and swaps for mechanics that need it.
Is bullshit.
"Litany of raid wide options", Dark Missionary and Heart of Light are literally the same CD and that is the only raid-wide option that exists.
HoC is a better co-tank button and so is Aurora vs Oblation because they're much more consistently useful due to strength and use cases. TBN has to risk not breaking beyond tank busters, and you don't spam it because you hold Dark Arts to be expended it for your 2 minute window and you want to have 9kish mana going into the 2 minute window, so spamming TBN is restrictive for not enough gain.
Paladin is the one with the most raid-wide options because they get Divine Veil AND Passage of Arms. Literally no tank can beat Paladin in raid-wide options.
Comparable dps to what dps? The Dancer? Can you tell me where on
this chart Dark Knight is comparable to any dps? Notice also how the dps difference between GNB and DRK is about 12 dps and the difference between WAR and DRK is less than 200. This is while Drk has shitty problems like current Blood Weapon and is the worst at actually tanking, and has the worst Invuln for Invuln cheesing because Living Dead is dumb.
It is one thing to say Drk isn't useless, but Drk isn't some perfect job either and what it is good at matters to about none of the average playerbase unless you're a speed run autist.