Putting aside how absolutely mind-boggling it is to me to assert that you should reasonably expect people to just suck up and live with knowing that their loved ones are locked into a fate worse than death for eternity - I suppose that's still a matter of personal perspective, no matter how much I may boggle - and the loaded questions regarding the specifics of the sacrifices to which nobody has any actual concrete answers - there is one thing brought up that does have an answer in the text:

Quote Originally Posted by EaraGrace View Post
All these solutions are hypotheticals that we have no evidence are possible, but let’s assume you could just gather enough aether another way. Why then, did the Convocation decide to use the souls of the living, if such alternatives exist? Did they just go for the easy option?
Because at the time they managed to summon Zodiark and stabilize the situation, everything else was dead. The planet was on fire. The waters were poisoned. The wind had ceased to blow. There was literally nothing left to use except Ancient souls. That was the entire reason for the plan to gradually nurture other forms of life back into the planet until it was "bursting with vitality," so "a portion of it" could then be safely swapped with the trapped Ancients.