Ultimately, the entire argument in defense of the Convocation hinges on the belief that the souls being sacrificed to Zodiark are less valuable than the dead Amaurotines. There is no mention of feeding Zodiark bales of hay anywhere in the game lore. And at the end it wouldn't have mattered what the sacrifices were, because we've already seen Emet destroy entire worlds in the name of his dead civilization.

I have no idea why you would want to reframe this as an 'ethical' debate when the Ascians had no pretenses of taking whatever they wanted in the first place.