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

[linayar]

But the idea of the Ancients have such powercreep that the Sundering was caused by the entity that effectively created you is interesting in both good and bad ways. Good as a twist but also bad in a "are we the baddies" kind of deal.

Whether we are the "baddies" is a matter of perspective, and according to the story's pov, we are not.

And I also think it kinda cheapens the overall story quite a bit. It's a whole lot of "To save the world I had to destroy it".

I don't see how that cheapens the story. Rather, it puts into perspective that Venat had faith for our future over the past that the unsundered wanted to reclaim. And it's quite clear that Venat sees us as heirs of the unsundered, presumably helped in part through our interaction. So sundering the world is not really destroying it, but merely creates 13 other possibilities in addition to the original.