Sundering the Nibirun

[Anvaire]

It was, otherwise the story makes no sense.

I dont think the Sundering was required, It was one way of achieving the solution to face meteon. I.e they needed a creature capable of weilding a mix of dynamis and Aether. They needed the ancients to not keep sacrificing to acquire what was lost. There were more routes to that end, the Sundering was but one possible outcome, ironically in many ways the route Hydealyn chose resulted in more Sacrifice than the ancients because while the ancients sacrificed lives to save etheirys, Hydealyn sacrificed everything else. Culture, capability, civilisation.

Another thing to note is that both of her solutions depended on Zodiark to succeed and the backup plan in reality was doomed to fail (using the moon to flee)

So when looked at with this guise, Etheirys was only saved because of both Hydealyn and Zodiarks Actions, but there is a case to be made that regardless, Zodiark was alwways required. While Hydelyn's actions were not. (Assuming the Ancients figured everything out and turned away from the whole sacrificing more lives things, which Elidibus would probably have done given his role as mediator.)