I tend to disagree on this one!
There were many fights where only the tank could survive...
On some games we had "tanks" with learning jobs: On each level up you increased stats from that job, so you was able to learn physical meele classes to get many Life Points while when leveling many mage classes you could get many mana...
On other games it was definied from the job bonus itself and the armor you could wear. Bonus on resistance or materia you add...
When the tank survived he could use reviving items or potions to heal up (mostly first the healer). While the healer uses grouphealing (in preperations for the next attack to heal up again after the attack) the tank revived the other party members with items. now when the healer is rebuffing the group (protect, shield, and so on...) the tank may helps with items or skills to survive another incoming attack. because the tank was mostly the HERO with highest stats i had let the mage help with items and rebuffing/healing stuff while the tank hits a strong attack on the boss. Thats called "recovery" in terms of MMOG and thats what many players miss in FFXIV. True its not easy to add in a MMOG but lets have a look at Crystal Cronicles multiplayer or Grandia II - its not impossible, even Final Fantasy games where we could deactivate the "boss is waiting", we had such systems...
My guess is they did not add recovery because the massive problems with lag and syncing players action, even now we still have massive desync in game.
Other situation, when a mage survived (some bosses did insta kills but only single target - never ever use insta kill AOE mechanics, SE - NEVER EVER!)
You could survive fatal blows with using mirror or "jump" ... there were some skills if you timed it right. its harder than when the tank survived but still it was possible to recover from near wipes. I just wonder why we have that stupid weakness in instances? Sometimes it is said: "then the fight will never end". Thats why you just have a max item size of 99 - i can remember fighting Omega in FFVIII for more than 12hrs and running out of potions... we had some challenges back then with not using special items and lowest level possible, but thats anohter story
Back to topic: Please dont name it DRK then, it is not a DRK if not using his own Life - just a sidenote: Thats why we have buffs like stoneskin and such to help DRK, but well FFXIV doesnt stick on the basics and weakens many skills.