Billionaires also aren't the problem. They are the symptom.

The problem is the system of economics and politics, by extension the various institutions they manifest, that allows individuals to become billionaires through the exploitation of other people's labor via private ownership over the means of production that should rightfully belong to the people through collective ownership.

Yep. A million dollars is much closer to 0 dollars than it is to a billion dollars. An even bigger problem is the ultra billionaires.

Aside from that simplistic argument, billionaires are much much more parasitic to society than millionaires are.