You provided zero evidence for your claim that millionaires “are almost certainly going to be a net benefit to our society.”
Plenty of millionaires are sociopathic, corrupt, and power-hungry people. The millions may even be a testament to this fact.
I don't need to. Just look at your immediate surroundings and ask yourself "who are the millionaires responsible for these products I enjoy?". Every component in your phone, computer, the chair in which you sit, the building you are in, the clothes you wear, and the car you drive. As a rule, people get rich because they create things, not because they exploit others. That's how capitalism works and it does work