Since Legion release it is oblivious that after Broken Shore events most of the work to defeat Burning Legion was done by Class Orders and its champions. Third side factions - not by Alliance or Horde. Although both factions did they part during early BL attack prior Broken Isles events and during Suramar campaign.
So now looking at new Alliance vs Horde war in BFA i'm wondering - was Jaina right after all? Should have Varian dismantle the Horde back in MoP siege of Ogrimmar? Its not another Horde hate thread - i'm just curious. What would have happened during Legion and after it?
We know that Alliance and Horde did the smallest to defeat BL. Even more in Stormheim they clash between each others. So argument that without one faction another will completely lose to BL and Azeroth would fall is kinda invalid.
So lets imagine Varian did dismantled Horde:
Orcs - I believe they would have been forced between choice placed in camps like after Second War or exiled into Outland.
Trolls - Considering their numbers i think Alliance would have left them on their own.
Taurens - Without the doubt Alliance would have offered them opportunity to work together and with time taurens would become full Alliance member.
Goblins - I think they would stay neutral like another (forgot the name) goblin cartel.
Blood Elves - Same treatment like with Taurens
Forsaken - The most interesting part. I believe Varian would put them ultimatum. They leave Arathi, Hillsbrad, Hinterlands and most part of Silverpine and Tirisfal Glades for Alliance. Sylvanas and most of Forsaken would ofcourse refuse and Alliance would put them down. Those Forsaken who stay away from conflict will probably ends up living like hermits and left alone.
So what next? No WoD since Varian would took Garrosh and executed him without trial. Sooner of later Burning Legion would returned to Azeroth. But instead of two factions who still fight each others there will be strong and powerful Alliance of 9 races - Humans, Dwarves, Gnomes, Night Elves, Draenei, Worgen, Pandaren, Tauren and Blood Elves.
Almost forgot - no stupid BFA.