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

[Cassar]

I think I would have preferred it if going to Elpis created a divergent timeline and we defeat the Elpis Hermes and Elpis Meteion in Ktisis Hyperboreia and save the ancients in the Elpis timeline but our timeline Meteion is still a danger. Elpis Venat does whatever an Elpis Venat does but apologizes for the actions (or lackthereof) of Eorzean Venat/Hydaelyn. So we return to our Eorzean timeline with newfound knowledge on how to deal with Eorzean Meteion and confront Hydaelyn on her inaction in the past and she gives whatever her reason is for her actions/inactions and tests us in battle like how it is currently.

Isn't this still a possibility? Is there any confirmation that the Venat we met in Elpis is the same Hydaelyn we fight against in present day?

Remember the Y'shtola quote:
"The implications of temporal magicks are not entirely understood, and so we cannot assume that our Hydaelyn and the Venat you met in Elpis are one and the same. Nevertheless, due to Her intrinsic qualities as an all-powerful being, I'd wager that Hydaelyn possesses the knowledge we seek."

Not necessarily. Imagine you take a bunch of twine and start laying it on the ground in a line. Now say somewhere along that line you decide to split it so you can run two lines parallel to one another. These two lines are still one before the split, yes? If you then grab twine at the starting point and give it a tug, you'll find the entire thing is yanked instead of just one of the two split off segments. You might also consider the example of igniting the twine at the starting point. That flame will in all likelihood consume all of it, not just one of the two lines you've made.

And yeah, I understand what you mean here... Temporal paradoxes truly are complicated to deal with, but you've convinced me of the Graha tia thing. But I'm STILL not convinced on the Elidibus thing! I truly believe he meant what I said earlier, he even goes as far as to explain the reason why we're not capable of altering the course of time "For the reality you wish to save - the reality to which you must return - exists as a result of the Final Days. You cannot reshape the past to undo the tragedies of the present. Cannot unmake the sorrow and suffering fated to come."