I think this is a wonderful write up! I would like to add, that many of the themes presented here are also seen in Tactic Ogre's Terror Knight, which I'm convinced the Heavensward Dark Knight is partly inspired from. Especially considering its greatsword usage, tankiness, and even the blatant armor set from the series itself. This supports your argument however, especially with the emphasis on draining and enfeeblements. While their counterpart tank, the Knight, used Phalanx to cut their damage down to single digits and created a wall that units could not move past, Terror Knights exhibited an aura of fear and caused enemies stats to dramatically decrease by being hit, or even just by being around them.

I think a lot of the reason people want Scourge, Delirium, and Reprisal back is because it created a stacking effect for debuffs on an enemy. Even though Scourge is denoted as "just" a DoT, it was cool to be able to stack effects for uptime. I too would like to see this stacking effect be more incorporated and used for things such as a draining DoT or other debuffs. I feel this could be possibly incorporated in the future through usage of Dark Arts, instead of it being mostly just a 140 potency damage buff button, something even Samurai has with more complexity.

Thank you again for the interesting analysis!