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

[Iscah]

Here comes the 50000 gil question.

Both Elidibus and Hydaelyn speak of a promise we made so long ago. Elidibus does not know to whom he made the promise and what it was. My WoL can't remember a promise either. Somehow I have the feeling that Elidibus and also Hydaelyn have promised Azem something.

Elidibus
What's it!? I have my mission!

Elidibus
I am Elidibus! And it is my duty to steer mankind and the very star upon their true course. This I swore to...to someone.

Elidibus
We spoke, and I swore...what? What did I...?

Hydaelyn
Though I knew failure after failure, by recalling thy tales---and my promise to thee---I found the strength to carry on.

Hydaelyn
Fulfill our promise. Right the wrongs committed when the world was yet whole. Silence the song of oblivion...

What have we promised ? I watched so many Cutscenes but there is no promise.

I would have to go over our conversations with Venat to answer that question regarding her (I would guess either our conversation up on the walkway or just before we return to our own time), but I don't think it's related to Elidibus and his promise.

I also feel like a lot of people have latched onto the idea that "Elidibus will promise us something during the Pandæmonium storyline" when the resolution seems far more simple and presented within what we have already seen.

He took up the seat of Elidibus, and at that time would have made vows to the Convocation to uphold the duties of the seat. And then everything happens and now his memories are fading. Look at those quotes you included: first he forgets to whom he swore the promise, and then the promise itself. He is becoming a shell of himself, with no memory of why and what he is fighting for.

I'm still not really keen that they twisted his original role in the story into this – it rings a little much of the writers not being sure how he could be defeated, so they undermined him instead – but for what he is in 5.X it does a good job of showing how much his mind and memories are crumbling.