Thing is content is naturally more physical heavy than magical, think about it, how many enemies use only magic? Ramuh, Leviathan's head and A4(s) boss. In these fights DRK will excel over PLD cause PLD brings nothing of value to the table, but in every other fight its reversed, DRK has nothing to match up to the shield, 30% parry rate increase vs. 60% block increase + 100% block every 30 seconds. Block is stronger than parry in every way (rate and strength). If a mob uses physical attacks then any magic attacks are usually just specials making STR down more useful than INT down for overall damage reduction and if you have a MNK then you get both. And now PLD will also bring just as much dps as DRK so it doesn't have that anymore either.

DRK will still be a capable tank, it will be able to handle tanking content using its universal mitigation and dark mind for magical specials, but it will be rare that there's a fight where DRK excels over PLD and WAR. Rune Fencer in ffxi struggles with the same issue.