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

[Garet]

When was it ever stated that the Zodiark plan required anything more than the 3 sacrifices we currently know about? Zodiark has existed for over 10K years sustaining himself solely on the prayers directed at him, and not once was it even suggested that his protection was waning overtime, atleast not as far as I can remember.

They were planning to sacrifice more to zodiark before Venat's faction decided to create hydaelyn to counter and stop zodiark and his followers. Emet even said that the sacrifices would not be in vain because they are hoping Zodiark can bring back all the ascians that were sacrificed to it. That was why they wanted to create the rejoining, so that they can continue where they left off and power up zodiark to defeat the whatever the finals days was and bring back their people to life.

Edit: They actually showed the scene of Ascians, in their despair, sacrificing themselves to zodiark in the hope of averting the final days which finally made Venat sunder the world. I think they create that scene specifically to show one part of Venat's reason of sundering the world.
https://www.youtube.com/watch?v=qlA0lGrSRqM

This is why Emet said to our character at the end that the "past they were looking for is not the future we are building towards to" Emet understood already that their way was not the best way to defeat the final days and for the future. But he is duty bound to do what he planned to do before as the holder of the title Emet Sech. But deep inside he knew we were the best chance against the final days hence even back in shadowbringers he helped us against Elidibus to "put his friend to rest" and put his trust on us.