You're kind of using selective reasoning here.
Either it's worth having an argument about good writing vs bad, or it's not, and then your original point that validating Sylvanas without payback for Teldrassil is bad writing doesn't matter.
There is nothing to be gained by redeeming Arthas. He had a successful, complex arc as a villain which would be ruined if it gets retconned into some tragic puppeteering, and there's nowhere for him to go after Shadowlands. The Forsaken taking him would be a bad joke, he has no claim to Stormwind, and there probably won't be any focus on undeath for a while. He could be the rightful king of the Scarlet Crusade, but then he'd be a villain all over again. Unlike Illidan or Kael (or Sylvanas), he is not a popular antihero hijacked into villainy for the sake of a random raid. There is absolutely no reason to bring him back other to satisfy the sickening fantasies of some very fucked up individuals who get off on maximizing Sylvanas's degradation. IMO, anyone who champions this particular dumpster fire of a narrative is disqualified from calling
anything bad writing, even Med'an and Season 8 of Game of Thrones, and should really look in the mirror and question themselves.
Edit: Besides, if you can say Arthas wasn't in control, you can just as easily say Sylvanas was under the effect of some magic of hopelesness cast on her in the Maw.
The medium of the game works on stopping world-ending threats, not on "getting revenge", particularly since former Sylvanas fans have little reason to get on board with this fairly recent (3 years at most, out of 18 the character has been around) hijack into extreme villainy (yes, Sylvanas was always a ruthless bitch, but she had a sympathetic emotionally wounded side too, and never went as far as in BtS/BFA prepatch) perpetrated by a couple of writers who are relatively new to the franchise. Not everyone is a Worgen player.
Yet we were never shown this transition explicitly, and the story played itself like it had depth and missing information. The "unfair world angle" is incomplete, since we don't know what supposedly makes it "unfair", so it sounds like a mopey emo teen's whines when they have to do their homework. If the true reason fails to engage players in any way, playing it as a secret drawn out ov.er 5 years would be just awful.
The fact that Sylvanas appears as an absolute villain can be a matter of perspective. People keep projecting what her ultimate goal is based on appearances, but we don't truly know. It would be a rather narrow minded approach to criticize the story if it turns out that the scope of what's going on with the Shadowlands is so vast that the fate of a city pales in comparison to it