While that maybe true(even the Bronze said the humans would fight each other without the Horde arriving) that doesnt neccesarily mean the humans would suddenly start genocidal campaigns. At worse it would be campaigns of conquests as oppose to slaughtering for the sake of slaughtering. Nor would they necessarily involve Kalimdor.
Also, depending on whether said Legion would even exist in AU Azeroth considering we do end up dismantling it, a war may or not even occur anytime within a the span of time we have been active in Warcraft.