Philosophers in the 18th century wrote about the danger that a group of rich people would corrupt a democracy and convert it into an oligarchy. The US constitution was designed to try to prevent this process, but it is has been happening for 40 years in the US -- and in many other countries, including Russia.

Russia illustrates that such an oligarchy can easily convert into an autocracy ruled by an empire. The cheater demonstrates it in the US. In ancient Rome, Julius Caesar was one of a group of three oligarchs, and when he had defeated the others and was making himself emperor, even killing him did not prevent the fall of the republic. I suspect that that was because property ownership had become too concentrated, making it very easy for a rich few (we could loosely call them "billionaires") to dominate everyone.