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

[Lauront]

The sundering was certainly a means to prevent Zodiark to be used to restore their society, and I believe that is their main objective. But that doesn't mean that is their only reason, because their group's objection to the sacrifice was also pointed out in the story. If the sacrifice itself was irrelevant, the focus would simply be on their objection to the restoration of the society.

Even Venat herself didn't simply say to restore their society is wrong, but to sacrifice more/others to restore their society is bad. (I don't remember the exact wording here, but I'm pretty sure she mentioned the sacrifice.)

The objection only comes up initially with Hythlodaeus's shade, and it's not until Anamnesis Anyder that we get the underlying motivation behind their position. Her arguments are clearly articulated with sacrifices with a specific aim in mind and which she labels bad for a specific reason, i.e. because they are (according to her) a sign of weakness (again, she even grants that the Convocation in enacting this wish is doing so out of a desire to safeguard the star's future well-being...), and why? Because she sees it as them not accepting their tragedy. This is all with Meteion's report on the fate that met a society which did away with suffering in mind. I wouldn't say she is on board with restoring their society. She is exhorting them to incorporate their lessons from the tragedy and to accept suffering as a constant companion.

The dialogue bears this out:

Bitter Ancient: This is all wrong... Why must we suffer so?
Fervent Ancient: It needn't be like this. No, there must be a way to restore things to the way they were. To reclaim the perfect paradise we once had.
Venat: No, my friends. Suffering exists, and we cannot pretend otherwise.
Venat: No civilization, however great, could eliminate it. If we would live, we must accept it as our constant companion.
Venat: Let us not seek to forget this tragedy. Let us carry it in our hearts, that we may grow stronger and know true happiness.
Bitter Ancient: We can't accept it! We won't accept it! It will be ours again─a world free of sorrow!
Venat: No, it will not, for there has ever been sorrow. Mankind was but spared its biting sting for a time.
Venat: So please, open your eyes. To try and reclaim those lives we lost by sacrificing yet more isn't wisdom. It is weakness.
Venat: No paradise is without its shadows. If we cannot accept this truth and learn from our pain, then our plight shall be repeated.

While I don't see an issue with people speculating that they may have had some moral reasoning attached to this (I certainly don't see it and I don't think we can even arrive at such a conclusion without knowing what was being sacrificed), all the texts where her or her group outline their motives do so with recourse to this ultimate aim, i.e. avoid their eventual doom. She even confirms this when asked by Y'shtola as to why she chose to sunder her people - it was to deal with Meteion (and I'd add to that the fate which met the Plenty.)

I believe the reason why we possess their souls is simply due to reincarnation via the lifestream. That is why I'm not certain that the sundering actually killed anyone, but rather split off part of their aether into the reflections, with those on the source still remaining as the source or the original/main copy. Then, due to the loss of aether, the Ancients either die off eventually, mutated/evolved as a race, or even mate with some of the new life to get the existing races.

It may not have killed them then and there, but at that point we're talking about an instant death versus a delayed one, like a poison or disease would inflict. And to a being that is virtually immortal, that "delay" may not take long at all.