What lore do you ignore?

[Iscah]

If we're going to count Venat letting Emet-Selch go as her being responsible for the deaths of those killed in the Rejoinings, then what about when Emet-Selch himself doomed Elidibus and the whole plan he and Emet had worked 12,000 years on by letting us free when Elidibus had us dead to rights and was on the cusp of knocking down the last barrier to bringing back Zodiark? By that measure it was Emet-Selch who killed off the Ancients, not Venat. If Emet-Selch wanted the Ancients to come back, all he had to do was to do nothing but his involvement was the final nail in the coffin that killed the last Unsundered and relegated the Ancients forever to history.

This is another point where it's important that they acknowledge modern humanity is a continuation of the ancients.

Emet's story arc is that he progresses from rejecting this prospect, to accepting it at the end of 5.0, to actually choosing to side with "new humanity" rather than "old humanity" at the critical moment because if he has accepted it as a basic premise then the next step is to ensure that the winner of this conflict is the one best equipped to carry some form of humanity forward.

Far from being on the cusp of defeating the "last obstacle" to Zodiark's revival, with multiple rejoinings and resolution of the Thirteenth still to go, what Elidibus is on the cusp of is his mind unravelling completely. If he had defeated us at the Seat of Sacrifice, he might have lost the last threads of his memory soon after, and would be in no state to continue the work to revive Zodiark himself while also having wiped out the other viable option for ensuring the ancients' legacy continues.