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

[Theodric]

Said civilization had zero qualms about throwing away half their population twice over and were perfectly willing to commit genocide of every new life born into the world just so they wouldn't have to feel sorrow or despair and recluse themselves into ignorant bliss of "paradise". The rest of the Ancient population making decisions aren't of high standing moral character either.

I think that particular portrayal is rather disingenuous and doesn't really align with the established events. For starters, the first and second round of sacrifices were willingly given and actively supported. Had they not happened, Etheirys would not have recovered from the Final Days.

As for the third round of sacrifices it isn't explicitly highlighted what the 'new life' entails though if it were the creations of the Ancients themselves, then it would be the equivalent of arcane constructs. Which is what they were in comparison to their creators.

I'd strongly suggest completing the side quests in Elpis if you have not done as much already. They don't exactly paint a picture of the Ancients being callous, cold and uncaring. They show emotion, they show concern for the Warrior of Light (despite believing him to be a familiar) and they also clearly show concern for each other and their creations. So I disagree with the idea that they aren't of 'high moral standing'.

I'd also note that the Sundered engage in far more active and open depravity. From slavery, to attempted genocide, to actual genocide, to theft, to murder, to wiping out entire species for trophies and trinkets to all manner of other things. All of which, I note, came about as a consequence of Venat's decision.

[TaleraRistain]

As for the third round of sacrifices it isn't explicitly highlighted what the 'new life' entails though if it were the creations of the Ancients themselves, then it would be the equivalent of arcane constructs. Which is what they were in comparison to their creators.

This is from Ere Our Curtain Falls, the same story I referenced in my previous comment

"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. "

So, this new life had souls and they wanted to sacrifice it to get back what they had.