While I get what you're saying, please keep in mind Arthas beat Illidan like a redheaded stepchild. They've retconned the lore at this point quite a bit but when Illidan runs off to outland he's a whipped dog. He spends his entire time up on top of the Black Temple engaged in chewing over the cud of his loss to Arthas, cowering in fear of Kil'Jaeden and the people running the show for the Illidari are the Blood Elves.
As to the OP: Yeah, Sylvannas may have gone to Bastion for her devotion to Silvermoon, but she never got that far. She's unable to even ascend to the Afterlife until her suicide in Icecrown and has generally been playing the Horde since that moment. So while it seems that Denathrius doesn't wreck the Arbiter until about midway through the activities of Legion (since warbrave Ordo is in Bastion), Sylv has certainly been pumping a lot of souls into the Shadowlands by helping to stoke wars on Azeroth.
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Pretty sure she knows about it and agrees.
Her fight is against the full system of death itself. Of the cycle of life, building anima over that lifetime and then the semi-parasitic nature of Shadowlands itself as it removes that anima from the souls that are sent to the various realms of the afterlife.
If anything she longs for true death and it is denied to her by this system which imprisons us all in a cycle of rebirth.