I don't think that's le Guin's thesis. The Dispossessed is her book that explores the nature of utopia, its subtitle is "an ambiguous utopia" even. I think the focus of omelas is the juxtaposition of outward peace and prosperity and internal brutality. The key to the book is how people react to seeing the rotten core of their society. It's not a thesis on what all societies look like-- le Guin has constructed many complex, lifelike societies that don't have such brutally evil defects, but a kind of society that sounds awfully familiar. The point of this story is how one reacts to the revelation of the society's nature, both within the narrative and externally in the reader. Who walks away from Omelas?