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

[Seraphor]

Given the the ancients were exterminated, I don't see how any justication can be put forward that isn't evil.

Oh my dear god.

[jameseoakes]

Oh my dear god.

?
There culture was completely erased, the individual rent down to at best there souls and forced to form new bodies if that's not genocide what is it?

[Seraphor]

?
There culture was completely erased, the individual rent down to a best there souls and forced to form new bodies if that's not genocide what is it?

For one, you're taking things too literally.
A literary justification is not the same as justifying the act of a character within the piece of literature.

But additionally, more literal to the story, the Unsundered by their committal to the Zodiark route, were consigning themselves to death, dooming their entire civilisation and the whole planet of Etheirys, by relying on a deity to solve their problems and give them a free win. Their incapability to face their own hardships was dooming them to certain destruction.

The only hope of survival, was an action that, through way of a causal time loop, was given to Venat as a sure-fire way to survive for at least another 18,000 years, and the only way to potentially survive past that.

The fact that it required ending life currently and starting it anew is a failing of the set up that was started 10 years ago without a set conclusion.

[jameseoakes]

The only hope of survival, was an action that, through way of a causal time loop, was given to Venat as a sure-fire way to survive for at least another 18,000 years, and the only way to potentially survivor past that.

It was not a surefire way to survive, all Venat knows when she causes the sundering is Zodiark will keep the world safe and when his shield is removed all hell breaks lose. There is no way of know that would could get to her and stop her and no way to know the ancients couldn't either. The game couldn't be bothered and instead just othered the ancients which was some pretty vile writing and then crowns the monster that murdered them a god and spends nearly all of EW telling is how wonderful she is.