Since Rithy laid all their hopes out, I'll take my own crack at the same.

Something that has always bugged me about Dark Knight is that it was never a conventional choice for a tank. Throughout the series it's been the epitome of the glass cannon, and was even a DPS in FF11. Obviously 14's devs are not beholden to 11's design philosophy in terms of choosing jobs -- just look at NIN actually being a DPS here rather than a fluke evasion tank -- but that role predated 11 by over a decade.

I have to keep thinking about why they chose to implement DRK as a tank back in Heavensward, and I keep coming back to two things about its historical precedent:
1) Dark Knights always used their health as a resource, and effectively became more powerful as they willingly allowed their health to get lower. Metaphorically speaking, "health as a resource" is the tanking role.
2) Dark Knights have often supplemented the above using either Blood equipment or Drain magic (such as Dread Spikes) in order to create a sustainable cycle of both supplemental health and damage output. This is rather meaningless to a DPS outside solo content, but once again, very meaningful to a tank.

And right now, the job accomplishes... neither.
I guess you can say Living Dead is a broad interpretation of the former point now, but before 6.1 I wouldn't even have qualified that as "growing more powerful" when they died since it was a death sentence for 7 years. Through a certain lens, "sacrificing TBN to burst your entire MP pool on Edge of Shadow" could qualify as sacrificing your survivability... if TBN wasn't presently being used to prefer MP overcap before burst.
Abyssal Drain is on a cooldown that severely limits its sustainability, having been relegated to a rather disappointing version of Equilibrium or Aurora that returns next to nothing on a single-target. Since the advent of Nascent Flash, in many respects, WAR has been and continues to be a better Dark Knight by the standards of older FF games than DRK... and just to really lay it on thick, at the same time NF was introduced, DRK's rotation was dumbed down to a knockoff of WAR; juggling the same buff timer mechanic, using the same resource gauge, cloning several of the same cooldowns including its immunity, and not even getting a branching combo out of the mix.

In interviews, it's been specified that 14's DRK is meant to be "the Magic Tank" -- hence its use of magical barriers and Dark Mind -- but if that was the goal, it seems like something from the Spellblade spectrum with the Runic ability would have been a more appropriate choice in the long run, such as the Rune Fencers from FF11.
Of course, by the time they made the choice to shift DRK over purely to a "magical" focus it was too late to rename it, but even so, the job's actual use of "magic" has largely been phased out in favor of flashy weapon strikes that feel neither impactful nor particularly helpful. Even when using spells in combat, both DRK and PLD suffer from the separation of Skill Speed and Spell Speed, so the insistent use of "magic" is arguably to its detriment, especially with how niche focused "magic resistance" has been. Combine that with the same double-weaving issues as GNB and the aforementioned WAR rotation... and it feels like DRK tries so hard to be different from all the other tanks that it fails to find its own footing at something it does both uniquely and successfully.

It just doesn't feel viscerally like a Dark Knight. And as someone who didn't pick up the job until Shadowbringers, I can't even say if it ever did.