Plato knew many things
Socrates died. Democracy killed him. That was the turning point in Plato's life. Despite tremendous wealth and an aristocratic background, he scorned them and turned towards the path of truth through dialectics, discussions, and debates, as his teacher had taught him.
He witnessed the demise of Socrates, the course of democracy, and its ill effects on the state. He was always wary of it and remained so throughout, criticising it. He suggested that the state be ruled by the wisest and bravest men for the betterment of the state. He knew how democracy corrupts the minds of the people and sides with the popular collective mind, which most of the time favours the majority and leaves truth blurred and badly harmed.