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

[Lauront]

*snip*

I agree, but I think it's worth at least addressing it as a point because it is her stated reasoning to Y'shtola, and it at least has a veneer of plausibility if you don't consider the fact that the story allows for some alternative possibilities right off the bat. Given how Yoshi articulated the point, you're right that the fate of the Plenty is the overarching concern and her decision to sunder is driven by and subservient to that:

Q: Venat had good intentions and her plan worked out in the end. But as a result the world was Sundered and most of the Ancients suffered. Was Sundering the star really the only way to save it?

A: This is a question that I consulted with Nacchan (Natsuko Ishikawa, Scenario Writer of Endwalker) to come up with the answer so it will make sense when we explain it. At the very least, as Y’shtola theorizes, Venat believed that the Ancients, being so dense in Aether, could not control Dynamis. So she thought they could not have stopped the Final Days and its source. So you know there were other Ancients who thought summoning Zodiark would solve everything but she saw that summoning Zodiark and using it to deflect Meteion’s “Despair Beam” and thought, “even if we were to do this and keep going as we are the rest of the Ancients will probably be unable to change as a people” when she’s looking at Hermes, or “we will always be our own undoing”. If you look at the dungeon, “The Dead Ends”, at the very end there’s a boss called Ra-la, and that’s sort of our vision for what probably would have happened to the Ancients if we just let them continue as they were. So for that reason, she chose to Sunder the star to dilute mankind’s Aether so that someday they might be able to use Dynamis and to fight back against despair and the Final Days at the Source

With that said, she did not really give them much reason to change, so it became a self-fulfilling prophecy.

EDIT: To put it another way, assuming that Venat's goal was to "save lives" is the wrong premise to begin with, and is going to lead to confusion because her actions don't make any sense with it. Venat's goal was not "saving lives." It is, as she stated, "proving life is worthy to exist," the altar upon which she allowed countless number of individual lives to be sacrificed. Was it worth it? Well, uh, come to your own conclusions. I don't think life has anything it needs to prove, personally, but.

The way I make sense of it is that she felt this question had to be answered one way or another for life to persist without meeting the fate of the other stars Meteion had encountered. My view on it is that while the issue may have been genuine, Hermes's conditions of "fairness" and the way she seemed to honour these in her own way by not revealing the actual basis of her concerns to her people (yes, the story gives some purported rationale on her part for it, but I don't find it awfully compelling) were utterly unreasonable and had no place in the entire ordeal... not to mention in the end the sundered get given the answers with plenty of assistance. Plus given that they share many of the aspirations of the ancients to reduce suffering along various dimensions, most prominently showcased in the Scions, there is little to guarantee that with the passage of time, any "lessons" learnt will not be forgotten in due course... and "our plight is repeated." Particularly given that these aren't the long-lived ancients.