Ventus would have died, meaning that Xehanort would have decided to use one of Eraqus' pupils to forge the x-blade instead. This means he would have visited The Land of Departure and things would have gone as they did in the game except Ventus would be excluded. That means that instead of Ventus, Aqua would release Aurora's heart from its prison, and Vanitus would have focused on making Aqua stronger instead of Ventus.
Once arriving at the Keyblade Graveyard, Aqua and Terra would be separated, and Aqua would be overcome by Vanitus, having already been weakened by Braig. The X-blade would be forged with no-one to destroy it, as Aqua did originally, and Xehanort would take Terra's body as before. Vanitus would come to assist Xehanort when Terra inhabits his armor and attacks him, and they would destroy the armor before summoning Kingdom Hearts, thereby causing the Keyblade War, and the end of the world by extension.
So in other words, Sora prevented the world's destruction at age 0. What a hero!