[Chien hesitates for a moment, and he sounds kind of uncertain. What would happen to those who didn't join them? Even the Avatars who were against Arca, what about them..?]

If it was that easy, there wouldn't be a problem. I think those that're against it will die, whether they're normal humans or not. Not all the Avatars are with us either. Lady Mathias ha only promised us Utopia, she never said what'd come after for everyone else.