Still the Warrior of Light in 6.x and beyond? [Spoilers through ShB]

[Kordarion]

The body horror in shadowbringers was what, one scene? And of someone we hardly knew. Second half of thavnair was pretty dark but ultimately short lived. Same with Garlemald. Didn’t help that directly after we’re having a lunch date with the scions. Compare it though to earlier 14 where the very first dungeon you do is the abode of a bunch of pirates housing kidnapped children and sex slaves, one of the 50 dungeons being about a woman who couldn’t deal with the death of her beloved so she turned to necromancy,body defilement,zombification etc to try and bring him back, ultimately cherishing his severed head.

If being short lived is a disqualifying characteristic then I would argue that both of your examples combined are more short lived than the second half of Thavnir. Let alone the other darker parts of Stormblood, Shadowbringers and Endwalker, for example Yotsuyu's childhood and teenage abuse at the hands of her family before being married off and latter being forced into prostitution, Yotsuyu's rein of terror in Doma, including but not limited to making family members kill each other to prove their loyalty to her, Eulmore and eating the flesh of its own sin eaterified populace without knowing they were doing so (Soylent green), the WOL being corrupted into a monster by the light aether they absorbed with the intent of saving a world, the existential nihilism of all of the dead races in Endwalker, Endwalker bringing up the eventual heat death of the universe and the finite nature of all life, Fordola massacring her own troops to try and halt the alliance during the second half of Ala Migho, and Meteion's quest to permanently kill every living being in the universe just to name a few.

Of course if we look outside of FF then we can find franchises that are even more grim dark than those, Halo with a collection of genocidal fanatics trying to kill every last human and the flood, a terrifying body horror plague that corrupts all living beings in the universe and that was only stopped by first wiping out all life in the universe to stop its spread, and not to mention the grandfather of grim dark Warhammer 40K. It has the works, four gods determined to flood the universe in either blood, plagues, hedonism or treachery and mutation, a hive mind hell bent on consuming all life in the galaxy, mid controlling space communists, the orks whose only purpose in life is to fight anyone and everyone, plus countless species who only want to destroy or enslave humanity just to name a few.

tldr, FF14 can get grim dark but it is a drop in the ocean when looking at what grim darkness trully is.