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

[Absimiliard]

Right but when Elidibus says that, he says it right before mentioning that you wouldn't be able to interact with anyone (I probably should've typed the whole quote) so, in my interpretation, what he meant was:
"Oh I'll send you in a state in which you won't be able to interact with anyone... However if, for some unknown reason, you are able to interact with anyone (which we do), you still won't be able to effect any changes"

And yeah, I mostly agree with the second paragraph of your post but I really do think that everything that happened in Elpis would've happened regardless of our intervention, and I find it hard to believe the writes wouldn't account for something so trivial such as the death of the WoL in Graha's timeline, which they made such a big deal about in Shadowbringers

That differs greatly from my interpretation. I saw nothing anywhere in that part of the game to indicate Elidibus' statement was accounting for changes to the WoL's state brought about by the extra aether. If anything, I should think the WoL's initial inability to do anything other than move around and listen as compared to later when you are doing such things as giving Venat a detailed account of events-yet-to-come, fighting great beasts, and even becoming the one directly responsible for Aurum Vale serves as fair contrast. That last one... that was slick on the writers' part. As much misery as that place brought to players prior to its revamp, and it turns out it's our fault.

There's also Pandaemonium to consider. I'll spare spoilers, but suffice it to say what the WoL does in Pandaemonium is most definitely effecting change. The story is set up such that it outright could not take place without the WoL present, up to and including an NPC that had been tipped off as (and directly contributed) to your character's coming. It would be rather peculiar for this to be the case if the WoL could not make changes in the past. Which, incidentally, Pandaemonium is accessible only because Elidibus sent the Wol to Elpis.

To the second point: They made a big deal of it in Shadowbringers, I'll grant, but outside of that context it has become largely irrelevant. The original timeline has no bearing on its offshoot at this juncture other than whatever memories G'raha may yet retain. The original timeline likewise has no bearing on whether or not the cycle on which the game is predicated maintains itself, given what we've been shown so far. So, if that timeline is now functionally irrelevant, it's doubtful they would need to account for it. Not gonna lie, I'd love for some content to send us there even if but briefly.

[Cassar]

That differs greatly from my interpretation. I saw nothing anywhere in that part of the game to indicate Elidibus' statement was accounting for changes to the WoL's state brought about by the extra aether. If anything, I should think the WoL's initial inability to do anything other than move around and listen as compared to later when you are doing such things as giving Venat a detailed account of events-yet-to-come, fighting great beasts, and even becoming the one directly responsible for Aurum Vale serves as fair contrast. That last one... that was slick on the writers' part. As much misery as that place brought to players prior to its revamp, and it turns out it's our fault.

There's also Pandaemonium to consider. I'll spare spoilers, but suffice it to say what the WoL does in Pandaemonium is most definitely effecting change. The story is set up such that it outright could not take place without the WoL present, up to and including an NPC that had been tipped off as (and directly contributed) to your character's coming. It would be rather peculiar for this to be the case if the WoL could not make changes in the past. Which, incidentally, Pandaemonium is accessible only because Elidibus sent the Wol to Elpis.

To the second point: They made a big deal of it in Shadowbringers, I'll grant, but outside of that context it has become largely irrelevant. The original timeline has no bearing on its offshoot at this juncture other than whatever memories G'raha may yet retain. The original timeline likewise has no bearing on whether or not the cycle on which the game is predicated maintains itself, given what we've been shown so far. So, if that timeline is now functionally irrelevant, it's doubtful they would need to account for it. Not gonna lie, I'd love for some content to send us there even if but briefly.

So to summarize: Zodiark and Hydaelyn do exist in Graha's timeline even though the WoL never went to Elpis there. Therefore if the story of Endwalker is that we were the ones that caused Hydaelyn because of our intervention in Elpis, then that would be a plot hole - and so I really want to believe that we didn't cause any meaningful change in Elpis.

When it comes to Pandaemonium... Yeah, it's true that what we're doing is most likely effecting change, but I would prefer to wait until the story of that is finished before commenting on it for sure. I think here's where the fan theories come into play.

[Absimiliard]

So to summarize: Zodiark and Hydaelyn do exist in Graha's timeline even though the WoL never went to Elpis there. Therefore if the story of Endwalker is that we were the ones that caused Hydaelyn because of our intervention in Elpis, then that would be a plot hole - and so I really want to believe that we didn't cause any meaningful change in Elpis.

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.

In other words, the WoL from the deviant timeline going back to Elpis could also maintain the original timeline despite them having died in it. They do after all share the same "root," if you will. Through this the predestination paradox remains intact, and the 8UC timeline continues on to whatever future awaits it after the death of its greatest hero while "our" timeline follows its path. In all probability the split in the timelines is part of the paradox anyway, which if true would make it a necessary brick in the road leading the WoL to Elpis in the first place.

Temporal paradoxes are generally obnoxious by default, but dealing with a causal loop and some aspects of quantum multiverse theory at the same time just really turns that knob up to 11.