Without the horde the nation's that eventually make up the alliance would have been easy pickings for the Legion. Yea sure the well was blown up and Azeroth saved. But that was like cutting off the hand to save the body. Not to mention, the void was in the back pulling the strings and apparently death too. All with a target on Azeroth.
As much problems and war the two factions have gotten themselves into, their constant strife has made them stronger to face threats.
Not to mention. The dungeon opening the dark portal confirms this too. The void wanted the Legion to come, to win, to try and dig and then blamo.