What's the point with that quite lackluster story in Endwalker?

[TaleraRistain]

The Ancients couldn't create souls, so there's no reason to believe Zodiark capable of that feat either. However, souls being randomly generated, it stands to reason there were some in the mix. The Ancients weren't vegan and they didn't consider returning to the star to be a bad thing, so they wouldn't have had a moral quandary about sacrificing a portion of flora and fauna to release their loved ones' souls from Zodiark purgatory.

They did have a moral quandary as Ere Our Curtain Falls outlines

"The people were divided, unable to decide what to do with the future that now stretched out before them. Many wished to trade the new life which had sprung forth to reclaim those lost in sacrifice to Zodiark. No small number, however, insisted that the fate of our world should be entrusted to those selfsame freshly minted souls. All were at our wits' end."

It doesn't seem likely that they're going to debate entrusting the world to flora and fauna. Like you said, they would sacrifice without hesitation. But what would give pause? New people.

Also....why sacrifice at all? Their people are lost. It's very sad. Move on. Humanity learned how to do this after the Sundering. They couldn't.