generally speaking, huge retcons are not taken very well.
Illidan wasn’t retconned. What happened at the Black Temple happened. It’s just that at the same time he was opposing demons he was opposing us, he was always the same moral quality he’s always been - grey. The only retcon was “he didn’t die lol” because of immortal demon hunter souls. It’s cheesy as heck but plausible.
You’re asking to not only spare Arthas his final death, but to rewrite the presentation of his lore so that he wasn’t in fact responsible for his own evil, but someone else (who?) was? The presentation is Arthas is pretty consistent, he was the one who took steps towards beocming the LK, and upon acquiring the position, became the dominant personality. Arthas, himself, the person, was the bad guy. If you write that away and just “lol he was corrupted by something lol” you completely trash his entire story and the significance of it.
How about, instead of butchering the lore, demand Alliance characters get more badass writing in general? You don’t need Arthas to have a badass alliance leader. You just need focus to shift away from ANduin and Jaina. I mean what about Genn? Genn is decent, without completely trashing the lore.