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

[Absimiliard]

Oh of course, how could I forget. Hythlodaeus famously is a person not well versed in the ancient's culture so his interpretation is very untrustworthy especially when themis' off hand remark does nothing to contradict his claim.

The ancients were down to sacrifice downright everything to stop the extinction of the planet, given her information they would have agreed. You are correct that she would have had to disclose everything and that's precisely why she didnt. The risk of hermes turning again was too great.

1.) Or, and here's a thought - maybe the EN text misconstrues a few things? Because it does. In fact, the EN version of the game misconstrues a lot of things, with the white robe being one of those things.

2.) At this point, we've had more of her motivations revealed to us. Her chief motivation was to shoot for enabling the WoL's timeline, while her second was in fact to to end her own species due to strongly believing they would end up like what players were shown the Plenty to have become. It had little to nothing to do with the planned sacrifices to Zodiark as others will so often posit. In other words, even without Hermes present, they would never have been told.

Even putting the above aside, the Convocation only had say in Amaurot, Elpis, and any other Amaurotine outposts that may or may not have existed. Getting them onboard wouldn't mean the entire world is also down with it, not that they would've agreed to start with. The Convocation's primary directive is the wellbeing of those beneath him, and gambling everything on a future that may not even be possible to reach not logical. It might've been possible to convince a minority - as shown by Venat having followers already - of the need to sunder the world, but the majority of people, ancient or not, aren't anywhere near altruistic enough to cease to exist for the sake of something that might not be possible. Sacrificing to Zodiark, meanwhile, at least guaranteed long term success (so far as they were aware) and meant eventually being resurrected anyway. Big difference there.

People seem to forget the ancients were... well, people. They possessed all the same emotions and emotional needs as their sundered counterparts, albeit most of them had more self-control. Their fears, motivations, and desires were mostly the same. Their willingness to sacrifice for one another far exceeded that of the ancients, but they weren't without a sense of self-preservation. Desperation will push people to go well outside their usual comfort zones, but even the desperate are going to choose the thing they perceive as being more likely to give them a beneficial outcome over an obvious gamble.