Ephemeral Equality

If Plato’s city of words is an attempt at representing or defining justice, then what does the city’s inevitable degeneration suggest? Are we to assume that this city’s inevitable degeneration, and the categorical explanation of how the city could degenerate, are simply aspects of the city of words that further our attempted understanding of justice, or could Plato maybe be insinuating that justice itself is only temporarily attainable, and inevitably unstable, instead of a fixated and definable concept.