DRK has actually always been tank-like, with their self-sacrificial nature being a tool rather than their defining concept.
As early as FFIV Cecil's ability, Darkness either doubled his damage output for 3 turns at the cost of 10% of his HP for each attack or gave him an AoE attack that costs HP because it was calculated like his high physical attack, compared to Rydia in the early game, Cecil is a beast. His 3D version, which doubled his attack was also great since it worked when a boss was vulnerable and left him available for item usage.
The next time we see DRK it's that weird bastardization that was in FFXI. Probably where a lot of the current problems stem from actually. The crying of scythes I still hear from time to time. (Since when is a scythe a good weapon?) FFXI basically looked at DRK in FFIV did Cecil's one shtick to the extreme: HP to attack. They also added in some low level black magic spells.
And then in FFX-2, we finally see DRK as what it should have been way back in FFIV if Cecil actually stuck with it. DRK is at its strongest ever here, and I really wish more people liked this game because the way they show certain jobs off from a gameplay perspective is masterful. DRK here is not only durable, boasting immunity to many common status effects, but also has a suite of potent black magic spells and two ways to sacrifice HP: Darkness for group damage, and the more powerful single-target Charon.
And since FFX-2, we've had FFT: War of the Lions introduce DRK as a mobile heavily armed fighter that sacrificed a bit of Knight's (PLD) durability and support for a set of devastating actions with equally devastating debuffs attached. While Bravely Default and Breavely Second both use DRK in its default setting, durable, but risk-taking.
To sum up, DRK has never been soft, their self-sacrificial skills exist as tools for them to use, while their sub-theme of debuffs has been ignored. It's like because FFX-2 bombed, they've been afraid to reintroduce DRK has a debuff/buff focused Job like it was in FFXI and FFX-2. Instead focusing on the risk/reward gameplay that frankly, does not work in an MMORPG as XI "artfully" proved.
This, a thousand times this. Class/Job identity should never be about "having something unique" it should be about "having something to give in a unique way." I think a good example would be Machinist and Ninja. Both are dexterity based Jobs with high burst phases and group support. Both offer an invaluable damage boost. However, Ninjas give their boost for a mere 10s every minute at a higher potency. While Machinists give 27-30s at 6% every 2 minutes. Both are great to have, so much so that for speed clears, people take both. Which is a problem that could be fixed as simply as making a boss immune to vulnerability up for a little bit, but that's a different topic.
Or take Summoner and Red Mage. Both offer party-wide damage boosts (Embolden and Devotion), both can raise downed allies, and both deal sustained damage with burst phases. Yet they're very different Jobs, since Summoner loses out on single target healing and rapid resurrections for stronger damage, not to mention the gameplay differences.
We can even see this with Paladin and Warrior now to a minor degree. Paladin has a broader support suite, but sacrifices some damage and a bit of personal mitigation to have that support suite. Warrior meanwhile has a very small support suite, but makes up for it with their damage, higher personal mitigation, and their one big support move having the ability to be exceptionally strong, but leaves them vulnerable afterwards.