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

[linayar]

Well no, there is a reason, and that reason is if their society was restored as it was, which those sacrifices were intended to accomplish (=restoration of the star to its pre-Final Days state), she believes they're still on track to meet an eventual doom along the lines of the Plenty, hence the way her rationale is presented is that the sundering is to remove the temptation of restoring things as they were. The objection is very clearly centered around this, i.e. the aim and not just the method. You can acknowledge the intention but in the absence of knowing what was involved and whether they'd even stick to this plan if they were given a full accounting of what had happened, it's a rather speculative acknowledgement, and we're stuck with her stated rationale.

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.)

It's a fine as an assumption but we must bear in mind that the sundered possess the souls of sundered ancients (as per Elidibus in 5.2), of which some 25% or so of the remaining population were still around at the time. Still, it's possible the bodies were the result of creation magicks, at least for some races, but who knows.

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.

but it was by their own choice, unless meteion influenced it? I don't see it as something bad, I guess

Setting aside for the moment the question of whether suicide is right or wrong, Meteion used that as another support for her conclusion that death is a preferrable outcome and decides that end for everyone else, and that is wrong.

The point is that, just because someone lost the will to live, doesn't mean everyone else would and should too.