Are you talking about damage?
Using FFlogs statistics, from Sigmascape until now, barring Phantom Train, DRK and PLD are more or less even at similar percentiles, with the differences usually being in the 50-70 DPS range. Even deep into the gutter tier of rankings DRK is comparable, and in Alphascape it is trending ahead of PLD in most 50+% rankings or at worst tied. Itemization this patch will squish the distance between WAR and the other two tanks considerably in damage difference. In terms of DPS, it's likely that DRK will be more than competitive.
If you're talking about utility then sure, it's true that DRK does less than the other jobs. TBN is not a particularly great shield for cotanks with its diminished effect on other targets, and it lacks raid-wide mitigation outside of a reprisal that every other tank has.
But you don't need that anyways. Healers, particularly Scholars, already have the market for unnecessary amounts of mitigation well in hand. Things like shake and veil are not absolutely necessary, and by all rights PLD has the better of the situation regardless with Cover and the addition of Passage (recently buffed for god-knows what reason).
DRK was in world first for both UWU and UCoB, and did not cost them either the clear (obviously) or their world-first-ness. They were able to complete the fights regardless. DRK not being able to provide extraneous under-utilized mitigation anyways does not matter. If you go as PLD over DRK to raid but never save lives or casts of heals with veil, or save someone's life with Passage, you may as well have gone DRK.
Warm feelings that you're playing the 'mitigation tank' are meaningless if you don't actually do anything to make effective use of it; saving lives or saving casts.
In terms of job complexity vs output, personally I don't find DRK much more difficult to play than SAM. This is of course a personal opinion about difficulty and doesn't actually say much about how you or anyone else feels about the job.
That said, asking that all jobs have a complexity that is directly proportional to their difficulty is just introducing a paradox of choice. If a job is complicated people don't play it regardless of how good it is. MCH is a good example, it's good (though I've disparaged and criticized its output in the past) but the requirements for playing it well are both asinine (rotational perfection not found in many jobs) and out of your control (miracle ping that puts you at the whim of God). The job has massive pay-off if played well, the damage is competitive and its output in rDPS is acceptable. But the barrier for entry is high, so few people play it. However, if you remove the complexity from the job you aren't making the job perform better or worse, you're just raising the skill floor.
It's a similar case with DRK, though without the starting 'is actually basically Meta' tag. DRK getting its complexity reduced does not help DRK. It only helps low% DRK players try and get closer to higher end DRK players that are playing the job well. DRK doesn't get any better with complexity removed, it just changes gameplay, and that doesn't make the job any better than it was before.
This is what happened with WAR in 4.2. The IR changes dropped the skill ceiling for the job to the floor, but did nothing to change how the job performed. It was still the tank with the best burst DPS, the best overall DPS, and with the shake it off change made it the second best tank (arguably) behind PLD. Reduced complexity was unnecessary IMO, but the reality is that changing WAR to make it easier did not make it a better job, did not make it so that you wanted it more. It maintained its status, it just had the additional benefit of having a lot more better terrible WARs out there compared to DRK.
DRK getting a complexity reduction improves its popularity maybe if people really were scared of that. It does not make the job more desirable in a party.
If you decide that there really should be a correlation between how complex a job is and how good (desirable) it should be, then you have to make a critical design choice: How hard can I make the best option? How easy should the worst option be? How much of a skill curve can I have at all before I'm just muddying how good a job is in practice? Does NIN lose TA because its not as hard as MCH and thus should be at least as good or does it keep it? Does BLM get better or worse because of movement restrictions? How do we balance jobs that are ultimately very comparable when we want to have some variety of different impacts on gameplay in terms of buffs or options (defensive or utilitarian?). Should ranged phys get harder by the virtue of having one of the best role skills (refresh)?