Everyone is stupid except me. I'm a misguided fool.

Yes he was. He assumed that the philosopher would know best how to serve the people rather than the people themselves. He failed to acknowledge that all people must have their own personal drives beyond seeking wisdom, with it being impossible for the monarch to be unbiased and driven only by reason. Reason has no drive of its own, never acting outside of servicing more base needs. Most monarchs will favor their own short sighted and selfish desires over those of the city.

Plato's political career failed so bad that he invented a reason for why only he should be allowed to do politics.