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

[Lauront]

Reading it over, I am not sure I fully agree with this point given the context of the story we were told. Zodiark was given form through "half of the planet Etheirys's population" to delay the final days. After that only provided a bandaid fix, in the cutscene we see what is presumably another half was about to given before Venat sundered the world.

It's less to do with it being a bandaid fix, and more to do with how badly damaged the star was - see below.

This was assumed to potentially continue till nothing was left.

Assumed by whom? It's not suggested by any source on the topic that it would continue until nothing was left. For example, Hythlodaeus's shade mentions two stages and a final one (at which point they'd resume stewardship of the star), which they had debated amongst themselves as per later sources, until they reached a consensus to go ahead with it (Venat's group excepted):

So I do not fully agree that however many "half of Etheirys" over and over again to be sacrificed to Zodiark is any worse or better than taking the life that would have been sacrificed and splitting it up in presumably equal parts across 14 reflections. Not saying one is right over the other, though sundering had to exist for some reason for our character to exist, just noting I don't think either outcome was preferable over the other, "damned if you do, damned if you don't".

Again, it's not either/or. Her own plan is based on Zodiark being summoned and restoring the star. So all those sacrifices are required for it. The sundering is on top of this. The third stage involved unspecified "new" life, implied strongly by the JP version of the same text to be the life seeded by Zodiark (I'd suggest opening this one in a new window/saving it to get round the sizing):



The texts are not specific on what they were, but Zodiark is shown to have the ability to realise creations via concepts readily - if these fit certain criteria, the star will bestow upon them appropriate souls, and they will constitute living entities. So it is quite likely, creations similar to those in Elpis.

The sundered are the surviving ancients. Not the intended sacrifices - and she only really opposes these because she believes if they were to restore the star and their civilisation, they'd end up like the Plenty - a function of her belief that they would not change.