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

[Teraq]

Oh boy, it's the famous mine shaft philosophy question in which Hermes sees a boulder fall down a mine shaft and goes "you know what? this boulder is EXACTLY what we deserve because we crush ants under rocks ALL THE TIME" and Venat, armed with all the knowledge to deviate the boulder entirely, replies "oh wow ur valid as heck, challenge accepted lmao" – like any sane person would. She was, after all, a wacky Ancient, who routinely carried out judgment on vast swathes of people with no prior concert or debate.

The part that precedes your post is huge here, I believe. The Ancients' plan after the summoning of Zodiark was to create a huge amount of life purely to sacrifice it to Zodiark in order to bring back their initially sacrificed brethren. THAT would be genocide, and on a scale that makes the remaining half of the population post-Zodiark-summoning look like a grain of sand in comparison.

What a very interesting claim. Can we see the source for the amount of sacrifices required and how it would "make the remaining half of the population post-Zodiark-summoning look like a grain of sand in comparison"?

In case you are going with "well Ancients obviously had a lot more aether than other animals and plants so they would have needed a lot!!!!!": as far as we know, beings in FFXIV are made of corporeal aether, soul aether and memory aether. As we now know as of Endwalker, Zodiark never consumed the souls and memories of the people inside him, else we wouldn't be having a little chat with Hythlodaeus, among others. This implies the aether that needs to be replaced here by the third sacrifice is just corporeal aether, to remake their bodies.

What proportion would corporeal aether be in an Ancient's total aether? How much would that equal in chicken and tree aether? I don't know. You don't know. None of us do.

The simple reality is that a lot of people were going to die no matter what happened.

People? In the third sacrifice? {Hmmm.}

I don't really understand in what way some people were expecting a better end for the Ancients.

Personally, I've never minded the fact that Ancients were dead. I do love my tragic villains. What I do resent though is the massive victim blaming going on in Endwalker, painting the Ancients as Honestly Kinda Dodgy Because What About Hermes's Little Hedgehogs? and kiiiiinnnda sorta deserving of their probably-avoidable fate because they would have totally wound up, in an indeterminate amount of years, centuries or millenia, like this distant alien civilisation we have zero context about (and neither did Venat as far as we know).

Meanwhile, Shadowbringers had a lot more nuance by not telling us what the source of the Final Days were, and making its antagonists relatable: i.e. wanting to bring their loved ones and society back, rather than lofty ideals about suffering and fauxlosophical musings about life and death.

Endwalker went and undid a lot of that: the Final Days couldn't merely have been an unspeakable, random tragedy that hit a perfectly good and normal people and drove them to desperate measures to cope, no – they low-key deserved it because muh playing god and dealing with grief the wrong way. Ascians, meanwhile, also took a big hit: their plea is no longer equivalent to the Sundered wanting to survive, because we now know their people was totally hubristic and wrong anyway because the plot outright tells you they couldn't have dealt with Meteion, or loss, or feelings. They can now safely be dismissed and we can all sleep soundly at night because their lost paradise totally wasn't worth it. The Sundered, though? Absolutely gucci! Only they could wield the Power Of Friendship required to resolve this convoluted scenario – but it turns out they needed assistance from at least three Ancients to result in the chain of events that brought them to the Endsinger, and without that timely assistance and information on the Final Days they would have succumbed to the apocalypse in even more horrifying circumstances than the Ancients did... so... I'm not sure what the point of it all is supposed to be about anymore, frankly.

The sundering had to have happened or the game just.. undoes itself?

Shadowbringers and its last Tales From The Shadow short story showed us changing the past is fully possible and would result in an alternate timeline.