You would actually have to crush the Tauren, Trolls, Goblins, Blood elves, and the Forsaken as well. The Forsaken would have no leash, so they would plague EVERYTHING. Blood elves have more skilled mages than Dalaran. Crushing the Tauren would cost thousands of lives as noted by Wrathion. As for the trolls and goblins I have no idea how long they would last to be honest. Goblins have lots of rockets. Less accurate, but just as damaging, if not more, than gnomish tech.
Wrathion only saw the Legion coming. The Old Gods and Azshara are still around. The world would fall apart.