LeGuin gets around the logistical problems of Utopia by leaving much of it blank. Aspects of it are either left blank or malleable so that we can fill in the factors that make up our own paradise. But beyond all things Le Guin is a realist. She anticipates our doubt. The beauty and the prosperity and the culture of this perfect place hinges in some undefined way on a price that has to be paid.
Someone pays the price for the greater good.
The price is simple. In a closet, kept close and quiet, maltreated and systematically abused, is a young child. The child might have had an opportunity for a full life if it had not been selected for this special role. As it is, the child is reduced to idiocy. It stews in its own foeces, mewling and whimpering like an animal.