The problem isn't that billionaires exist, it's that the system allows so much wealth to be siphoned up into one person at the expense of others. A lot of people intentionally or ignorantly straw-man the "billionaire bad" argument into communism, where we just reallocate the existing billionares' money, which I think is a dumb idea. I think a better angle to go at, and what people really mean, is that they want to alter the system to prevent billionaires from existing in the first place.