Endwalker was a great mess(SPOILERS)

[Driavna]

Right until EW and the discovery of the cure for tempering to me the reason of sundering was simple: primals drain the land from aether, the more powerful the more aether to keep him around. If left uncheck a summon will create stuff like the burn unless is feed regulary (and, pretty much that was the plan of the Ancient after stopping the end of days Zodriak will again be boosted to bring everyone back. After that another excuse to summon him again will be bring to the table and blablablabla). Also, any tempered is bound to want to keep her god around or summoning it once more, and since there is no cure you can't reason with them, only to put them down.

It's not without reason, knowing that Hydaelyn was less powerful and could not confront Zodriak directly, that sundering the whole star was a last resort since I doubt they will want to kill their own kind, now out of control due tempering.

Anyway, at least to me, FFXIV always had a plot that the less you think about it the more you enjoy it. By ShB it went for cheap emotional moments and during EW got out of control (the whole Venat cutscene after Elpis is that. Amazing if you don't think about it but if you do is a big mess).

Oh, one last thing: according to the whole main point of EW the Ancients were doom and low key the last dungeon show us their ultimate fate even without the final days. They could have made a better job delivering Venat point, about learning that suffering is part of life and having to deal with it instead of using primal copium to fix any problem and eventually fall to nihilism (or just like: hey our mission is done everyone back to the star!)