I'm wondering if it will be Settra that does Archaon in....the Gods do like their games!
And when you think about it, total dominion diminishes them, after a fashion. Beastmen are less favoured than other mortals, because they're born into thrall. Valued yes, but not favoured. If Chaos dominates, all mortals either perish, or turn to Chaos, and thereafter all will be as the Beastmen in terms of value.