dark knight plays best into raidbuffs in the game
dark knight benefits most from forced downtime of all tanks
dark knight is best in frontlines of all tanks
dark knight has the most magic mitigation
You probably do not remember before gunbreaker if you did not play before shadowbringers launched. Incidentally dark knight was not a dps tank in shadowbringers, paladin and gunbreaker were. (Shadowbringers's dark knight was where they introduced "warrior"-lite. Warrior itself being forced into the requiscat shaped burst when the 4.0 version flopped.)
The reason different tanks can't be good at different things - mitigating magic for instance - is that people will whinge incessantly when there's an all physical raid floor and need to swap over to paladin or have your party revolve around their inability to take physical hits. The move from job to encounter complexity is the final nail in that coffin, most jobs are only really allowed their identity in pvp kits.
It is clear that you don't like xenos, but you shouldn't let him live in your head rent free. He's not in every shadow.
Dark knight could probably do with a small buff to shadowbringer or living shadow, but given how hard it plays into raid buffs at the top end it would be incredibly difficult to balance- see the top adps tanks for both ew ultimates and pandaemonium floors prior to 6.4 (The 6.4 tank buffs even all say they're to bring them in line with "other tanks"). That being said, the only acceptable solution is to revert the whole game to 4.55 and pretend the last 3.5 years never happened. This would also address the healer issue and wipe the endwalker summoner from existence. A solution with no downsides.