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

[Vyrerus]

The only reason why you would need to create monsters to fill your ecosystem is because you've created monsters already

Not necessarily. With such grand regard given to the ecosystem, and the fact that all creatures seem to be given aether manipulation, I think it speaks more to the idea that even the unsundered world had naturally occurring monsters.

In other words, there's no reason to create monsters if there weren't monsters first. There's a whole section of history we do not have, such as pre-paradise Amaurot. Perhaps monsters were made when they still had war, perhaps our progenitors' monsters were born of their own progenitors. So maybe they're just doing the best with what they were given.