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

[dapperfaffer]

Yes but this was post final days. She easily could’ve told everyone before this and prevent the days from even coming to fruition. She instead kept it a secret. Even her Anamnesis dialogue suggests her followers didn’t even know what her actual plan consisted of nor that it would end in their own demise.

Telling everyone what she knew after the WoL involvement would change the entire history of the game and involvement of the WoL.
Pre WoL, Hydaelyn sundered Zodiark and created the Source and the 14 shards because self sacrifice to keep what the Ancients had was bad.
Post WoL, its the same thing, just with the added bonus of suffering is best for life to have meaning.
Lahabrea was right, the root of the corruption is Hydaelyn.

Which I don't think is the OPs point. I think its because much like ARR and other expansions the story wasn't centered around the WoL/WoD, it again was based on the Ancients and their actions. Which works to provide the exposition needed to end the total story of ARR and Hydaelyn but makes for a pretty boring story.