Not just the players, though. Malfurion knows about it, and at least a few faction leaders must have doubts, Velen for instance or the Highmountain. Plus we the player are supposed to be influential and important now. Why would the Archdruid of the Dreamgrove or the Highlord of the Silver Hand, who have decades of evil smiting under their belt and know exactly the dangers that face the planet, happily murder the opposite faction because miss zombie or the boy king say so?
I'd be fine if Horde vs Alliance tensions boiled to the point where localized conflicts happened (that could be the Warfronts, around important pockets of Azerite for example) while the ''main'' plot relies on investigating the planet's state and uncovering the mystery of Azerite. I'm not saying both factions need to hold hands and sing Kumbaya. I'm saying there's got to be a middle ground between Kumbaya and a world war.