Your father was both right and wrong. There is obviously a relationship between democracy and progress, as it's true that letting people do what they like to do seems to have an impact on ingenuity. It's no accident that ancient Greece gave birth to many things from theatre and natural philosophy to steam engines and analog computers. This, as you say, is owed in large part to the fact that much of the menial labour was carried out by slaves or low-ranking citizens, freeing up the time of a part of the population to think and invent. Later on, automation took on a part of this labour.