It thoroughly undermines democracy, unless we want to redefine democracy as 1$ = 1 vote.
“Power resides where men believe it resides. No more and no less.”
These billionaire are powerful only because we have collectively decided to put such high value on the product of their companies and investments. It all stems from perception.
Follow it through. People believe what they are told, to a certain extent, so Power resides where people are told it does. Who tells them where it resides? Those who can afford to shout about it a lot.
Thusly a billionaire is powerful because they have the ability to make people believe it by projecting that position through "propaganda", and this doesn't go away. Money buys people listening to you. By the quote, that is essentially power, and so money is power.