Let's play a game: Save the Ancients!

[Yuella]

And yet it was a victory nonetheless. Were it a war with a litany of casualties, what would you say then? Were it... dare I say, a form of pandemic created by a malevolent entity, what would you say then? The Ancients have Ancient-specific crises, for sure, but again I'm still seeing no argument that justifies why they're inherently more dangerous or at risk than mortals, who are more far more vulnerable and lack the same resources they had to address problems facing the planet in the case of a mass emergency.

No, it was not a victory. Endsinger was still out there. Her song was still a danger to the Ancients. They kept wanting to sacrifice more and more lives to restore what they have lost. The biggest danger was the ancients could anytime commit mass suicide by deactivating Zodiark to let Endsinger's song through (just like Amon/Fandaniel did in our timeline). The Anamnesis Anyder cutscene confirmed that Zodiark was a temporary solution. In order to save the ancients, it had to be before Zodiark was summoned. Once Zodiark was summoned, the sundering was 100% necessary.