Voss replacing Sylvanas is like Matthias Shaw replacing Anduin... only worse, because Shaw is actually more interesting than the boy king
The Horde is severely lacking first tier characters... we were lacking them since Legion. Only Baine, Lor'themar and Gallywix left as legit racial leaders, the rest core races left aimless or reverted to second tier chars... and Gallywix is presented as an antagonist too,
especially to his own race (something that never really happened with Sylvanas). Meanwhile, the Alliance has Anduin, Tyrande, Malfurion, Velen, Genn, Moira, Muradin, Mekkatorque and Jaina, with her family in tow.
Sure, you can count Thalysra and Talanji as newly introduced but heavily featured leaders, but then you have Turalyon and Alleria on the Alliance side, both major characters with deep, deep lore roots. You want to count Rexxar and Rokhan? Ok, but count Maiev, Shandris and Brann as well. And the Alliance has successors ready too, Genn has a daughter who was there from the start, and now we found Bolvar's kid as well. Even if we add Thrall returning from neutrality (he is a joke and a permanent mockery of a character now, but ok), the Alliance still has Magni and Khadgar waiting in the shadow.
What Sylvanas actually needed was a resolution with Arthas. She should have been part of the encounter back in WotLk, instead of getting a failed attempt that didn't progress the story in any way, and made her look even worse after the Wrath Gate.