Do you know who started the whole thing up? it was the old Gods. They were the ones who showed the Highborne(probably just Xavius) how to communicate with Sargeras. They showed Sargeras how to create the Demon soul, which was needed to open the portal, and secretly had in its function, to open up another portal to the Old God prisons. They also attacked Nozdormu during this, to prevent him from helping or i dont even know(honestly I think the time travel part was just to Mary sue Krasus and Rhonin)
But in all that, everyone was an Old God pawn. Deathwing betrayed and decimated the Dragons, Demons and night elves fighting, Demi-gods dying. Seems like this whole thing weakened anyone who could also put up and defense against them.