The Fates have warned the gods that going against AO's will is a straight path to doom. Sooner or later, his will will hold true, and his wishes will be enacted upon his universe. For even if both mortals and divines forgot his dominion, the universe still remembers its master.
Whether they are friends or foes, it was of utmost importance to keep AO's will in their minds and make sure to never go against it. The reason the World Tree immediately abandoned Dendron after Arad's threat was that the universe itself shifted.
Arad declared that he'll burn the World Tree, and it was just Kali who wielded her divine power to support his will. The universe itself immediately started rejecting Yggdrasil's existence, and the Fates called, warning the World Tree that her fate had shifted, heading toward direct destruction. If Arad doesn't burn her that day, or in the next ten years, he'll eventually burn her. No matter how powerful she gets, the will of the Overgod is inevitable.
