I think they (the writers) had tunnel vision on adding "grey" morality to the original Ancients, in order to contrast what Emet-Selch told us in SHB. Because graying the morality of Hydaelyn and Zodiark worked extremely well to SHB's benefit, so they figured they'd do it again. No, see, the Ancients weren't actually so benevolent that they sacrificed themselves so half their number can live...they just had twisted views about death. Zodiark's summoning wasn't JUST an attempt to save their star, but also an attempt to reclaim the "paradise" they thought they deserved. Just like in SHB, we were supposed to see this as a "no one is right -- every sun has its shadow" theme of moral relativism.
The problem, however, was that in SHB the "true villain" of the entire piece was a nebulous natural disaster. The Final Days at that point could have been a tornado or an earthquake, or space radiation, which makes the gray morality work because it was simply a force beyond control. But EW then not only gave the disaster a name and a face, it then upped the ante by attaching it to mankind's final boss: oblivion. And by doing that, they undid the attempt to create moral gray; because, by definition, anything that fights oblivion is objectively good. There's no other way around it because if oblivion wins, the story is over. The Ancients, unlike Emet Selch, weren't simply "wrong from our protagonist's POV" -- as far as the narrative is concerned, they're just flat out wrong. And, by extension, Venat is narratively right. What she did may not have been pretty or fair, but as far as the story's concerned, it was objectively the correct thing.
This explains why a lot of people liked EW's story up until Elpis. That story thread was the exact moment that any sense of moral gray or relativism was thrown out of the window. After Elpis, you either agree with Venat and the Scions and take the plot's lesson as it was intended...or you disagree with them and the plot starts to make little sense the more you scrutinize it.