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

[Absimiliard]

Please explain how the Elpis time travel makes sense. After all our intervention didn't cause anything, right? So why was the timeline Graha is from not destroyed by Meteion?

The WoL's intervention very much did have an effect. It's part of the causal loop. Elpis is set at a point in the timeline long before the 8UC (which I'll note is in fact the original timeline) and splinter (in which the game is presently set) divided. E.g. the WoL from the splinter going back to Elpis and maintaining the casual loop will keep the 8UC timeline going as well. Think of the 8UC and tangential timelines as being like two branches extending from the same tree limb.

As for "current" events: Zodiark isn't dead in the 8UC, which means modern Meteion is not yet a threat to that world.

Game's got plenty of plot holes, but I wouldn't count those among them. I do, however, believe the use of a causal loop wasn't the best decision.