I think that if you have to work harder, with a greater risk of personal loss or failure, the payoff should be proportionately greater. Its really simple. Working harder to attain results equal to, or in some cases worse, than the other jobs in your role, to me, is not fun.
That's the common belief, but that works poorly in MMO's where the dispredancy only lead to the hardest but more rewarding job beeing the top choice, everyone (good or bad) switching to it and neglecting and refusing the other jobs that are "not so worse" but considered inferior and useless.

People only wants to choices, even if they are miles over their ability to play, and since in FF the tank DPS matter for more than a reasonnable choice, it means if DRK is superior when player accordingly, most people will only want that job and none of the other (or another for LB purpose and choice of spells).

That's the "hair I'm cutting" here, you don't want balance. You want DRK to be better at the expanse of the other jobs, whether it is merited or not, it's simply not fair as is the current clunky situation.

While it used to be a credo in many, many games, the "I't's harder so it must be better in the end" credo ended poorly when people shared logs, guides, rotations, Gcd per gcd studies which ended making anything suposed "hard" an easy to learn bargain. No one will pick a lesser job because it's too hard for them to even waste time on the hardest job.

And because of some communication before the launch of SB, I don't think a hardest job will again be held as the topmost one.

For example purposes, let's look at the respective immunities.

Hallowed Ground; 10s; 7m CD
Holmgang; 6s; 3m CD
Living Dead; 10-19s; 5m CD

At face value they all are relatively even. However, once you look at the effects, you start to see the issue. Paladin requires nothing from anyone since it literally takes zero damage. Warrior, meanwhile, can self-heal if absolutely necessary. Dark Knight is the only tank entirely dependent on a healer to survive its own immunity. Say you're in a scenario where people have died, including one of the healers. Paladin and Warrior can use their abilities to save the day. Now Dark Knight technically can, but you've also put a huge burden on the one healer to heal you within ten seconds. Warrior just switches to Defiance, uses Equilibrium and Inner Beast to buy itself some time.

This is the issue when people talk about how unrewarding Dark Knight can actually be. Nearly every hallmark ability in their arsenal requires more work for only equal to even worse results.
Like you said, it's an example purpose. Because it's the worst one (healer depandancy, death if badly executed), the rest of the DRK toolkit is far from beeing that costly and problematic. LD simply doesn't deserve such a long cooldown, if they don't want to change how it works.

But should the DRK loose it's "costly nature" ? I think not, that would be playing another job.

Should it be best tuned ? Yes.

Should the job be better than the others ? That's not up to me either, but it will only weight the imbalance. I don't mind if the DRK is harder for the same results as others, but making him better will only cause the previously stated problem that cripples the game and the mindset of players.

Of course, everyone wants his main job as the better, and everyone allways have a wagon of reasons that seems plausible to anyone with the same mindset. But is it objective ?