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

[PawPaw]

nor was it ever reasonable for the Ancients to ever believe of their own accord that an unhinged bird-girl had flown into space to slowly kill any planet with life left on it.

Have to disagree with this part. No reason why she couldn't have used one of the many methods the Ancients have shown for reviewing memories. Very likely that they would not have believed her if she just told them without providing evidence but there were ways of getting said evidence.

[Theodric]

Her dialogue in Elpis expressed that she would avoid bringing the subject with Convocation members so it felt like she avoided them at first and did things on her own. At a later point, around the time of Zodiark's summoning she made contact with the Convocation to stop them from sacrificing half of the replenished life to bring back the first batch of sacrificed.
Also, the ancients were masters of creation magic, pretty much anything could be created from it so it's not so unreasonable especially if someone such as the revered Venat was to say it.

Have to disagree with this part. No reason why she couldn't have used one of the many methods the Ancients have shown for reviewing memories. Very likely that they would not have believed her if she just told them without providing evidence but there were ways of getting said evidence.

To be clear, by 'their own accord' I'm referring to them coming to the conclusion of Meteion's existence on their own. Venat should have told them what was coming to forewarn them, though she did not do that and instead left them scrambling about in a desperate effort to try and figure out why the Final Days were happening.

Then when they found a solution to the problems that the Final Days brought about, she decided to blame them for not finding the 'right one' and resorted to the Sundering. All without giving them any explanation.

In the end, it seems that the writers decided that the Sundering needed to happen no matter what, though they were probably hoping more people would just go along with all the cool, flashy cutscenes and not think on the implications of what the context and consequences actually entailed.

[Kyohei]

To be clear, by 'their own accord' I'm referring to them coming to the conclusion of Meteion's existence on their own. Venat should have told them what was coming to forewarn them, though she did not do that and instead left them scrambling about in a desperate effort to try and figure out why the Final Days were happening.

Then when they found a solution to the problems that the Final Days brought about, she decided to blame them for not finding the 'right one' and resorted to the Sundering. All without giving them any explanation.

In the end, it seems that the writers decided that the Sundering needed to happen no matter what, though they were probably hoping more people would just go along with all the cool, flashy cutscenes and not think on the implications of what the context and consequences actually entailed.

Ohh, sorry didn't read it that way. Then we're on agreement in the end.
The sundering needed to happen no matter what for the simple reason Vahlnir cited, in a purely "technical" view. The moment we got to know the present world was sundered in ShB sealed the fate of the game in the sundered state. Except the reasons it happened don't make much sense when looking closely with a logical approach.
Again, I'm putting this on the lack of overview and preparation from the writers.

They could have made a branching timeline however, in which Venat takes a more logical course of action, but maybe that's what we'll see in the additional story.