I find quasi or near genocide to be a bridge too far for a faction you don't want to case as outright villains in the narrative - Teldrassil as it current is in the game was a mistake on the writers' part unless they truly and fully want to define the factions as good (Alliance) vs. evil (Horde) as it was in the First and Second Wars. I think it would've been better for the Horde to get bogged down in Darkshore by the Kaldorei, prevented from realizing full victory in the War of Thorns. The Battle of Lordaeron would instead be a diversionary tactic by the Alliance to reduce the pressure on Darkshore and Ashenvale, drawing away Horde attackers to serve as defenders at Lordaeron. Sylvanas would then go ahead with the destruction of her own capitol to try to kill Anduin and the story would play out normally. Saurfang's rebellion would be based on discovering the full extent of Sylvanas' plans i.e. Teldrassil and Stormwind, and the growing sense that Sylvanas' idea of war would spell the Horde's destruction instead of its salvation. In this new context, events would play out relatively normally as they have.