For the people of Utopia, the Wasteland was dangerous.
If one couldn't understand what evil was, then survival in the Wasteland was very difficult.
The Explorers who had experienced the fairness, justice, and transparency of Utopia were at a loss when they arrived on the unfair, unjust, and opaque terrain of the Wasteland.
Of course, the nature of Utopia meant that anyone wishing to invade this city-state also had to abide by its Rules, which in turn would make them members of Utopia, nullifying the concept of an invasion.
Even a Saint had to be bound by the Rules of the city, and this was the reason Utopia would never fall.
Just as Plato had mentioned before, even if the choices of the populace led to their own destruction, as long as those choices were within the rules allowed by Utopia, they could not be stopped.
Because there will always be successors who step in here and continue to enact the perfect justice of Utopia.