It is Plato who affirms it, as Jean Bodin relates it, in his IVth book of the Republic: Republics die, like all things.
Republics suffer change by nature. So that Plato, not yet having knowledge of celestial movements, and much less of their effects, said that the Republic which he had ordered, and which seemed so perfect to many that it should be eternal, would take its change, and then be ruined, [even though it] did not change its laws: like all other things, he said, which are in this world. So that it seems that neither all the beautiful laws and ordinances, nor all the wisdom and virtue of men could prevent the ruin of a Republic.
Jean BODIN IV book of the Republic, chapter 2
Thus, no matter how well the Republic is constituted, and this is what Plato had undertaken, it risks withering away even without changing its laws.
→ 4 Quotes from Plato on justice (Book I of the Republic)