Originally Posted by
Coconut
Redeeming Arthas is bad for several reasons that do not apply to Sylvanas, as I have pointed out:
1. Because his finished story was already compelling, and this would completely negate it. Sylvanas did not have a coherent "fall into villainy" arc, it all happened off screen (ironically, after a soul-searching arc was promised in Legion). There is nothing to lose by negating it, and there is room to turn all the smoke and mirrors into a "misunderstood" philosophical twist. In fact, the narrative sort of demands the missing pieces to be filled, and filling them with what we think we already know would be plain old boring, which is a Doylist argument on why a twist would work best, but a strong one nonetheless.
2. Because there is nothing for Arthas to do after Shadowlands. Sylvanas can either stay behind as an agent of the Shadowlands, either as penitence or as an achieved goal, or she can return to Azeroth to get the Forsaken back on their feet and give the Alliance a reason to hate the Horde again - because if Sylvanas's goals were ultimately good, it would be in the Horde's nature to take her back, as seen with Grom in Warcraft III, the Zandalari in BFA, the Blood Elves after all the stuff Kael pulled off, the former Garrosh and Banshee loyalists, etc., but the Alliance would never forgive her. This hearkens back to the Pandaren starting zone and the philosophical differences between Ji Firepaw and Aisa - the Horde respects action even if it's damaging. I can't see Arthas returning to Azeroth, and having him take over a function in Shadowlands would be unearned pandering.
3. Most importantly, because a scenario in which Arthas is redeemed specifically through defeating Sylvanas would be extremely messed up. Imagine Tyrande goes crazy with Moon power, nukes Thunder Bluff, and Sylvanas redeems herself by showing up and killing her. Would you enjoy that? Or would you be outraged? Arthas could redeem himself perhaps by healing Sylvanas, but even then the first two points remain, and it's kind of lame to make the story about him.
Bonus point: If Arthas shows up to redeem himself, what purpose does Bolvar serve? They would both be former Lich King good guys dude bros. Fighting a mega bad guy Lich King dude bro. Kind of crowding the one motif there... Even if he returns as a paladin, he'd be crowding Anduin, Turalyon and the recently deceased Fordring. How many paladin human dudes do we need?
The only tasteful way Arthas could play a part is as a minor bit in Sylvanas's story. She could either save Arthas too, among others in the Maw, in her true goal twist reveal, showing that she is over the suffering he caused her and feels empathy even for his soul (emphasis on soul, as opposed to caring for how the living get into the Afterlife), OR she can show her cruelty by torturing a terrified child-soul Arthas into anima pulp, taking the revenge she was denied in Wrath, but in a petty, ugly way - in case she's meant to remain a villain.