The issue falls apart with the simple fact that Warrior is barely if at all in any real way portrayed with "tank" qualities, yet here it is in XIV as a tank. There are only a couple of skills that really qualify as "tank" skills, at least ones that really do that kind of thing. Healers and casters are more of a thing, but mostly it was slow but sturdy melees and quick but weaker melees as the other categories. The best you tend to get is just a really sturdy character with maybe some skills that help either draw enemy attacks or protect the party, but they usually have more to do than that.
Further, while Dark Knights work as a damage dealer, there are a ton of jobs that make sense as that. You could justify almost any job in Final Fantasy as one and it wouldn't be odd. Tanks and healers are a lot harder to do, but Dark Knight still has tank-like qualities, at least as much as Warrior does. XI is the only real oddball, but Dark Knights in other games are very sturdy. Having an attack that sacrifices HP for damage might seem odd on a tank, but it'd be worse on a damage dealer, so I'm not really sure what the solution would be there. If it was, you'd have Dark Knights killing themselves from the enmity they pull from the increased damage they're dealing. And if they aren't doing enough damage for that to be a viable concern, then it's probably not worth the HP loss to begin with. At least on a tank you're going to be healing them consistently anyways.
Finally, not at the last post but earlier, while Gafgarion's English title may have changed, his Japanese title remained the same between versions. He's still a Dark Knight, and was clearly intended to be, much like how the Holy Knight is effectively a Paladin, just different takes on the same concept.
