Dark Knight has just about always been a tank. In reference to FFXI, all melees had some sort of tank abilities, though obviously some classes were better at it than others. There was plenty of content that DRK could tank. The problem with FFXI DRK tanking was newbies using Last Resort and Soul Eater. Though not the ideal tank, it worked. Besides, FFXI was FULL of hybrids anyway and so most classes could fill multiple roles.

Seriously though, I could've told you DRK was going to be a tank in FFXIV years ago, I don't know why people were setting themselves up for disappointment.