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

[Valkyrie_Lenneth]

The Final Days did not wipe out the Ancients. It decimated them, certainly, though there were enough survivors to work at preventing Etheirys from being destroyed by summoning Zodiark to serve as a barrier against the return of the Final Days and also restore life to the planet.

It was Venat who wiped out the Ancients through the horrific act of genocide that was the Sundering.

So what was it when they sacrificed half the remaining population to summon zodiark?

Mass murder?

And she didn't commit genocide, the souls were simply split. Proof of that aplenty in game.

If you want to say venat was wrong, that's fine. She wasn't a hero. But neither were the convocation for sacrificing their own population. They could have left it after that, but chose more sacrifice.