This is a really good post.
I'm not sure how much substance I have to add to it at the moment, but I would like to throw an opinion on the fire here, speaking at least for myself: I think that it is totally okay for DRK to have complex interlocking mechanics, or for DRK to be reliant on specific conditions for their abilities to be effective, if and only if the reward for successfully mastering those mechanics and using them correctly is much greater than the reward other tanks get for using their more straightforward abilities.
I personally feel - and I don't think I'm alone on this one - that part of the draw toward playing DRK has always been that it's simply a more complex, nuanced, and risky class to play than any of the other tanks. I think that a lot of DRK players like the challenge, and like having to exercise caution and awareness to make full use of their abilities, and want the class to feel as though you're constantly on the cusp of losing control and having your entire kit fall to shambles, as long as the reward for not a mistake and not falling into shambles is that you get to feel seriously powerful in everything that you do.
I don't think there's a clearer example of this than comparing DRK to the other tanks in terms of their 4-man dungeon AoE rotations:
- PLD's dungeon strategy is to maybe Flash once or twice to hold aggro, then spam Total Eclipse and keep Circle of Scorn on cooldown, while rotating defensive cooldowns.
- WAR's dungeon strategy is to get Storm's Eye up while pulling, then spam Overpower for enmity and damage, use any BG you can get on your AoE BG consumers, while rotating defensive cooldowns.
- DRK's dungeon strategy is making sure that you've got the MP to secure aggro, then make sure you have enough MP to use TBN for defense, rationing any remaining MP between Abyssal Drain for damage/enmity, Dark Arts for DAAD healing, or Dark Passenger for burst damage and small mitigation, while also making sure that you don't overcap on Blood, using the Blood on Quietus for extra damage and MP management - making sure that you don't overcap on MP either - which drives further AD damage/enmity, DAAD healing, DP burst, while making sure that you have enough MP in the tank to use your next TBN, and trying not to have to resort to single-target combos, to keep yourself afloat. Strategies for using your various skills differ wildly in Grit compared to non-Grit, rather than just being a simple "same thing, but more/less tanky" difference, because of Blood Price versus Blood Weapon, and how Delirium interacts with either. Actual standard cooldowns basically function only to close the gaps in your active mitigation/defense rotation.
The difference in complexity, the risks you take with each skill use, the tradeoffs you have to make when comparing one option to another, etc, are many, many times greater than what you experience while tanking on WAR or PLD - playing DRK properly in a level 70 dungeon feels about as hectic as holding a controller in each hand and playing a WAR and PLD at the same time in entirely different dungeon runs. But the reward for playing DRK properly is felt very plainly: Your ability to mitigate and otherwise alleviate the healer burden is nearly as good as a Turtle Mode, 100% tank stance PLD or WAR, while you also deal as much or more damage than a fully-offensive WAR. The effort you put in to playing the class well is rewarded, relative to the smaller-effort, smaller-reward results you get on PLD and WAR. (WAR getting a stupid and unnecessary buff to Steel Cyclone's healing component throws this a little out of whack, and once again WAR is getting over-rewarded for not really having to do anything, but it still isn't enough to really upset the power rankings)
3.x DRK also came close to this ideal. It was a significantly tougher tank to play at a high level than PLD or WAR, with lots of limitations and pitfalls, but you were rewarded for doing it well: An optimized 3.x DRK did almost as much damage as a full-offense off-tanking WAR, while also having significantly improved defensive capabilities (barring a few fights where Holmgang being especially overpowered made WAR broken from a defensive standpoint). This wasn't perfect - the skill differential should have resulted in a situation where DRK did as much damage as WAR, not just almost as much - but it made learning to play DRK feel fun and worthwhile, because again, you were rewarded for your effort.
The real shame of 4.x DRK is that it's really not easier to play than 3.x DRK was - in a lot of ways, the moment-to-moment decisions you have to make, the amount of planning and foresight you need to have, the understanding you need to internalize of the various ways in which the class generates, consumes, and exchanges resources, etc, are actually more complex and difficult to deal with than the Heavensward incarnation of the class - you just simply aren't rewarded for dealing with those tradeoffs and drawbacks, or for mastering the class's nuances. Given the conditions you outline here, and the punishments you suffer for making a bad call, you would expect that a properly played DRK does 150 more DPS than a Deliverance WAR, has all the defensive capability of a Defiance WAR, and can protect the entire party as well as supplement a co-tank's mitigation as well as a PLD, and that these conditions, requirements, drawbacks, etc, are the only thing holding DRK back from ruling over the game's meta in a 3-DRK standard party composition. Instead, a perfectly played DRK is dead last in just about every metric you could use to measure a tank, and all of these conditions just punch it down even lower than that.
There's an audience out there for a hard-to-play tank with lots of pitfall skills and mechanics that can harm you more than they hurt you - but the performance rewards for playing such a tank need to be proportionally higher than what you get by playing the more straightforward PLD and WAR.