Having to press Dark Arts almost every other GCD or every third GCD does constitute spamming, especially compared to virtually every other job which have no such mechanic. Dark Arts used to be interesting, however now it's simply "press this button to do 140 more potency." Curve and Spit being the only exception. Actually, it's exactly as clunky because you only have one open GCD window to easily fit in a double weave without clipping. Meanwhile, WAR can do Fell Cleave/Upheaval/Onslaught/Fell Cleave without skipping a beat. It's a needless punishment on DRK—one rectified if they remove DAs from Syphon Strike.
And?
Warrior deals substantially higher damage, has party utility, which is infinitely more useful, is the only tank not punished in any way for aggro generation, has the most flexible tank stance and the best self-healing. It more than makes up for not having a single target shield by being better at practically everything else. And yes, DRK does have great snap aggro generation. Unfortunately, it's often rendered meaningless because aggro as a whole has become a joke nowadays. Therein lies the issues for DRK. The few things it's good at have very little impact. I won't deny TBN is a nice shield, however you simply aren't trading out WAR and PLDs kits just for TBN. Which is why DRK has warmed the bench since 4.0.
As for rotation. Warrior was already brought down to brain dead levels. Your entire burst phase revolves around spamming Fell Cleave. It's more simplistic to optimize WAR than PLD. Which brings us back to the exact issue I mentioned previously: DRK works twice as hard yet sees little in the way of results.
WAR has Unchained. It doesn't care about going into Defiance for ten seconds. Equilibrium is full minute faster and isn't depended on for damage. You mention it yourself. Equilibrium and Clemency are generally used as recovery tools. No DRK ever will hold Sole Survivor for a panic scenario unless it happens to be coming off CD as things begin going array. Basically, it's not reliable for such a purpose.
Yes, except it isn't better than anything WAR or PLD offer. I won't deny TBN is nice but it simply cannot compensate for the sheer versatility WAR/PLD offer. There's a reason DRK lags behind both its counterparts by a 4:1 margin. Even in Weapon's Refrain—a fight tailor made for DRK's strengths—sees it with only a third the clear rates as PLD/WAR. Party mitigation is far more useful than single target due to the predictability of tank busters. This makes Shake It Off, Divine Veil and Passgae better.
The whole reason DRK gets lumped with WHM and MCH is due to their respective counterparts. DRK may be in a better place than MCH/WHM, however it remains in a bad spot. I suppose, if you wanted to get technical. You could put it with SAM, which is decidedly the worst melee but still decent. Does it really matter though when all four jobs need some serious attention come 5.0? Even DRK needs less, it needs love because it's dying on the vine right now.