4 ideas
Every republic contains two humors: the nobles' desire to dominate and the people's desire not to be dominated. In Rome the tumults between plebs and senate did not weaken the state; they produced laws and institutions such as the tribunes that protected liberty, because each side's pressure checked the other. A republic that suppresses this friction for the sake of quiet, as Sparta and Venice did, gives up the capacity to expand and adapt.
Machiavelli holds that liberty rests on customs that make citizens obey good laws, and that where a people's habits have been corrupted, laws alone are very hard to make work, so a freed corrupt people tends to fall back under another master or needs an extraordinary quasi-royal hand to reform it. Rome is his contrast: it recovered its freedom after the Tarquins because its people were still uncorrupted, but could not after Caesar, when corruption had spread.
All states and sects decay as the founding virtue that animated them wears off. They survive only if something periodically drags them back to their origins: a frightening external shock, a law that enforces accountability on the powerful, or an individual whose exemplary virtue revives the original standard. Machiavelli puts the interval at roughly ten years, before memory of the founding fear fades.
Ask which group has the stronger wish to keep liberty. The nobles want to acquire power, while the people want only not to be oppressed, so the people make the more trustworthy guard. Seen this way, a multitude that is bound by law is steadier and judges better than a prince, and it is more constant, errs less on choosing officials, and recovers faster from its mistakes.