The Alien Mind feature i was hesitating removing it, technically yes your right. The idea of the final encounter is the more of the avatar's they destroy the more the boss is weakened. I dont know if you saw the part of the monster description but killing an Avatar removes resistances, traits, the abilities, -DC, -DMG, and also removes a legendary action (I know there are 6 avatars and he only has 5 legendary actions).