Hey peepz,
So the Nibirun are a stand-in for the ancients, they are what would have happened to them if they had continued their existence without changing course.
And the only way for the ancients to change course was through being sundered. Since they are physically incapable of managing Dynamis in a positive way their aether levels had to be reduced in order to have a chance to withstand the despair of the final days.
So unless we sunder the Nibirun in Ultima Thule, they will never be able to find peace. They are physically incapable. Unless they are not really like the ancients at all, which would never be stated, thus being a hard cop-out.
Because think of the implications otherwise: It would mean that the Sundering of Etheirys never would have needed to happen. It would mean that it is not one's physical makeup which decides their fate (the message of Endwalker) but that it's possible for anyone to handle despair (weirldy enough the direction in which the beast tribe questline seems to develop) no matter how dense their aether is. Venat would turn in her grave if she knew! Cause that would have been a massive doozy! Sundering a world over absolutely nothing.
So if the writers wanna be bold and consistent and not choose the cop-out of having the Nibirun actually completely capable of changing their mind through some nice food - thus forcing the conclusion that they are nothing like the ancients, they have no choice but to sunder the them. Leaving them be would have negative consequences for the Dynamis. Convincing them shouldn't work because of the way they are.
Also think about how cute a tiny sundered Nibirun would be! We could get one as a pet to follow us around <3