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

[tokinokanatae]

I don't think it's the same thing. I think the story was attempting to portray the Ancients constantly trying to undo their suffering rather than living on and learning from it, which supposedly would've led directly to them being unable to stop the Final Days should Zodiark falter. However, we were never actually shown that and just left to infer it from the potential third sacrifice, a few lines of dialogue in the last Elpis cutscene, and the last dungeon in the Dead Ends, so there's not nearly enough reason to believe that that's the case.

I agree that this is what the third sacrifice was supposed to imply, but the narrative undercut itself by having all those souls that sacrificed themselves for the planet existing in (an implied painful!) purgatory for thousands and thousands of years. 75% of the Ancients were basically being tortured and taken out of the cycle of life! Leaving them alone is like having proof that Hell exists and that you could save everyone suffering there, but shrugging and saying, "well, they chose eternal pain to save their loved ones, all we can do is accept that and move on with our lives."