On the ongoing question here about whether it's acceptable to commit mass-murder if the stakes are life itself in the long-term, I want to - putting aside my earlier comments about my main objection being how it's questionable to write a story like in the first place, especially when said mass-murder is framed as due to some inherent racial characteristic - take a bit of a different tack to Crowny and challenge this idea at its root.
Why, exactly, does potential future life have any fundamental value at all, especially when compared to life that already exists in the here and now? To paraphrase Kant, man is not a means to an end, but an end unto himself; conscious beings do not exist to affirm the world, but are rather its purpose, for nothing has any value except that which is imparted by the perceptions of a conscious observer. Those who are not born are (obviously) not conscious, and so have no inherent value. Therefore, a choice that will result in the world ending in 100 generations as opposed to 100,000 is at worst ethically neutral so long as that ending is not any more unpleasant for the conscious beings that have to experience it, because the only "victims" are unborn, unconscious ones, and in both cases their number is infinite anyway. If we assume the number of unborn beings "saved" by extending the lifespan of the world is, say, a trillion, then the total casualties go from ∞ to ∞ -1 trillion.
...or in other words, still ∞.
And that's if we give the fundamental worth of life the benefit of the doubt, which despite Endwalker's attitude, many philosophers don't. If extending the lifespan of the world would come at the cost of it being much more unpleasant to live in, than arguably not doing so is more ethical because it reduces the level of suffering per conscious being over the course of their lives.
For the sake of argument, let's also set aside all speculation about the Amaurotines finding another solution to the Meteion problem given enough time, and all objections to the story's weird ideas about utopias, and take what Endwalker appears to be saying at face value: Without Venat doing the Sundering, the Unsundered would have carried on as normal for a while, sacrificing some life that may or may not have been sentient to Zodiark in the process, before at some point relatively far in the future either becoming consumed by ennui and committing suicide, or getting killed by the Final Days after the Unsundered Zodiark finally breaks down in the face of Meteion's assault.
Even with this in mind, Venat's action feels incredibly morally wrong to me. It is a choice to indiscriminately sacrifice every currently conscious person against their will (and creating a bunch of essentially new conscious beings in the process), not to protect anyone who actually exists, but purely for the sake of the unborn.
How is this justified? Going past all of my more straightforward problems with the expansion's themes, this aggressive pro-natalism, this idea that the continuation of life has some essential value unto itself beyond the welfare and happiness of those who currently exist, really alienated me. It's frustrating because the text doesn't really engage with the obvious utilitarian counterargument at all. It just asserts repeatedly that the continuation of life is undeniably worthwhile for its own sake and strawmans any opposition as basically a kind of super-depression; perpetuation of life matters more than quality of life, and no matter how much your life is materially awful, you should continue to live anyway because shut up just do it. (Off-topic, but EW left me with an extremely dire sense of how the SE writers could approach topics like assisted suicide or the right to die, though in fairness, it's pretty common for anti-depression stories to fumble in these areas.)
As any individual has the absolute right to contribute to human extinction by choosing not to reproduce, so to does a society have the right to choose the lives of those who are currently alive over potential future life, so long as they're not explicitly damning people to suffer through their actions more than the alternative - as we are by destroying the planet through climate change while continuing to create a huge amount of new humans who'll have to deal with the consequences. Separated from the issue of the sacrifices, I believe the Unsundered had this right, and felt strange about the story's portrayal of this attitude as unacceptable.