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

[Lauront]

They absolutely care about us! I don't think everyone thinks non-Ancients are as worthless as ShB-era Emet. But... they ultimately see the animals as animals. It's sad when an animal doesn't meet its metrics and has to be destroyed, but it's just how it is. (They also seem to have planned to destroy Meteion - thus why Hermes grabs her and runs into Ktisis Hyperboreia, and says at one point that he needs to get the information out of her "before they're erased from existence" - but I'm not sure about that. Hythlodaeus and Emet don't object that no one wants to erase her. It is, however, possible they didn't think it would help. He didn't seem to be entirely rational at that point.) The entire structure of Elpis shows that they don't think living things have an inherent right to keep living. I think the "you have to meet your performance metrics to justify your continued life" thing also explains why people keep asking Venat when she plans on dying when you're in Elpis. She's done with her job. She's not meeting her metrics because she doesn't even have them any more.

Hermes was applying that logic to a group that it wasn't meant to be applied to - obviously no one ever intended for the Ancients to be required to justify their own existence to some external force on pain of death. But the logic itself didn't entirely come out of nowhere. If Hermes believes that the animals he works with are on the same level as his own people (which he seems to - I'm not saying it's rational or correct, but I think that's where he's coming from), then establishing performance metrics, a "supervisor" who can decide whether we've met those metrics (Meteion), and the usual consequences for failure (destruction of the entire species) is a logical extension. It's not a good one, but it does follow a chain of logic.

They are not mere "performance metrics" - they relate to the ability of the species to integrate into the star's broader eco-system, and it is abundantly clear from the lykaones why that is. As for treating animals as animals? Sure. But then I don't see the particular problem with this, especially since both the sundered and RL humans treat them far worse, in comparison to the ancients who even try and ensure they're put down gently if needed and where possible; and I'd consider Hermes's "judgement" as ridiculous even in the former two cases, nevermind the ancients. Hermes may follow a "chain of logic", but it's still one in service to a spite-driven execution that will encompass all life, and one where, as Amon, he is still not happy with any answers he receives.

That's the point of the Rejoining, though. You're a failed lifeform who shouldn't exist as you are, and thinking you want to live is presumably one expression of that. Emet wants to fix you and make you into the corrected version of you.

With the caveat that he is dealing with beings that exist as a result of genociding his people and thus also exist at their expense, yes. Enabled, of course, by Venat 'allowing' him to escape to fulfill her plan.

...they put flowers with them and were genuinely saddened by their deaths. They asked us how we say goodbye to the dead to incorporate that into that rite. I have no idea what you are talking about.

Neither does he, to be fair.