This is going to sound super biased but i don’t think Sylvanas is the issue here. (Not the root of the problem at least, she is indeed one still, a very big one).
As much as it might surprise some, i think the issue here is Anduin.
Its him the one that Blizzard wants to be always immaculate, he is the one that is unequivocally painted as the hero every time, every story (starting with most of the Alliance ones) gravitates towards him.
He is constantly validated, he must always be proven right.
There CAN’T be a “morally grey” war when you have the leader of one side that CAN’T do WRONG (I mean, he physically CAN’T now).
He is being groomed as the supreme hero on Azeroth, and this problem has grown far beyond the initial area of influence (the Alliance), to now encompass the rest of playable factions (the Horde).
Now, the story has everyone twisted and changed to fit in with the underlying message: Anduin is better.
Even the prelude for the expansion already pointed at this. Most of it is centred around him, while the rest are mere props for his majesty to shine. Even Sylvanas turns secondary when compared to him.
In game, things work similarly.
Dissenters are beaten into shape by turning the shadiest ones into full-blown villains that need to be eventually removed (Sylvanas, Gallywix,…), and the moderate into subservient pacifists beyond any logical reason (Baine, Saurfang,…).
What started off hinting at making every Alliance leader a patsy for the Wrynn, and seemed to doom them to play second fiddle for as long as whoever sat on Stormwind’s throne wished, has now devolved to the point EVERYONE on Azeroth has to dance to their tune.
That’s how you end up with the King of Gilneas playing the submissive subject to someone that is supposed to be his equal, or will probably have the Night elves turned into rash and irrational beings that need a good Wrynn finger-wagging (much like Dwarves had in MoP, Draenei mid Cata,…)
This isn’t the characters fault by the way.
Its basically certain writers that can’t keep their bias and fanboyism in their pants.
TLDR:
Yeah, Sylvanas is now Malificent. She has been taken out her place, and thrown into the spotlight, while having the worst of her multiplied by ten. She is the worst, and everyone else is dragged under her.
But how wouldn’t she? Her opposite is being thrown as the ultimate perfect being.
Of course everyone else will end up shuffled around and out their spot in order for it to happen.
Making certain Horde segments take the fall is something inevitable.
PS: And at this point, don’t know if having Vol’jin would’ve changed that. If this was the planned route, we would’ve probably had him turn random-evil instead of Sylvanas.