Gee if its so far off why are we fighting the last foces of the Void Lords NEXT EXPANSION? Just gonna sit on Azeroth and let Genn bitch about Forsaken for a few decades after we dismantle a demon army and titan and dread gods.
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Because it gives an endgame
An excuse to thrust the Legion forward and destroy them and still have azshara and Nzoth be considered noteworthy thereafter.