Really, asking if the Ancient society was perfect is doubly useless because for one, even if it was perfect, that doesn't justify the acts of genocide the ascians enacted in order to try and restore it.

And two, by the time of the sundering, the collapse of thier society was already a foregone conclusion. Even if the third sacrifice had restored everyone who intially sacrificed themselves to Zodiark, there would still be some form of ensouled life stuck in Zodiark now.

In that sense, Venat and her followers were not only Walking Away From Omelas, but making the city uninhabitable on thier way out too.