You're not taking into account the timing of these events, and I do not blame you because the timing is never furthered upon in a meaningful way, in lieu of the vague nothing cutscene we were given. The scene in Anamnesis Anyder was before Hydaelyn had ever been summoned, and thus, this single Ancient's opinion did not constitute what Elidibus said before his death, which was:
'This... Yes... I would become Him. I would save everyone. This I believed. Yet still they cried out, in rage and despair... Divided—over the fate of the star. A rare occurrence, always fleeting. But not this time. Not this time... Reconciliation. Elidibus. I was needed. I withdrew myself from Zodiark. For them... My people. My brothers. ...My friends.'
At this point a great many Ancients were split on this issue, it was not 'fleeting' and implies Elidibus realized he needed to mend the conflict after it had already been going on for a while—as in, after Hydaelyn had been summoned, not before. Hydaelyn was a living embodiment of resistance, and I believe it made Elidibus question the path they were on. Do you think the 3rd sacrifice would have occurred without the heart of Zodiark in control, as the last two did? I cannot see that occurring, and certainly don't see it as the inevitability that you claim it is. To say they were 'dead set' on pursuing sacrifices at this point is woefully inaccurate, when the person who is actually controlling Zodiark decides the conflict isn't worth it and withdraws himself from the primal, at the cost of his power (To a point where Hydaelyn is able to beat him). Again we are unfortunately lacking in a great deal of vital information, thank you Endwalker.
Also, if you'd read the very same novella you linked me, you would find that for all they knew, they were, in fact, erasing the past, they themselves even called it 'altering history.' That this is done in pursuit of changing what came before is inarguable, even Graha frames it as 'If history must be unwritten, let it be unwritten.' I fail to see how this is in any way different from what the Ancients were attempting to do, and who could blame them? Wishing for the better days of the past is normal when you are experiencing the literal apocalypse. That some would sacrifice unwilling life for that is wrong, but again, not every Ancient believed that to be right, and this resistance convinced the most important member of the Convocation at the time to step down. It was far more nuanced than what Venat's blanket judgement and cutscene could ever cover, and it goes ignored in service of hollow 'self-evident truths' given with no plan of action.