In your scenario the Horde would have an massive internal conflict with tens of thousands dead and the army being splintered and exhausted. Yet when the Alliance arrives in full force Thrall and a handful of Shamans would easily destroy the ENTIRE Alliance military forcing the survivors to flee in panic. It's a flat out defeat, a humiliating and emasculating one at that.
The Alliance would be worse off than the Horde, having been defeated easily by Thrall and a few Shamans alone. It would mean the Alliance can't hold a candle to the Horde when the Horde is at it's weakest, it would turn the entire faction in one big and pathethic joke. Thrall defeating all their military, Varian, Jaina, Tyrande and all the other strong characters they do have.
What's next? He rides on a giant sea wave to Stormwind and summons a volcano below the city burning it down before making the sea engulf both Teldrassil and Azuremyst Isles while using the Mountain to crush Iron Forge and you claiming that it would be a pyrrhic victory for the Alliance and give them a chance to actual depth in Alliance politic and character development?
You would actually manage to top Cataclysm that way, by a great deal. It would be completly unenjoyable for the Alliance half of the player base who don't even get to actually participate in the raid and it would be unenjoyable to a great many Horde aswell who actually want a strong and capable Alliance. I for one want an Alliance who makes for a great antagonist for the Horde (and vice versa), someone I can take seriously and "pride" myself in fighting. You'd make them a joke I couldn't take seriously the same way Cataclysm did just far worse.