If you believe that democracy is a good feature of government, then policy outcomes (such as, arguably, the existence of billionaires) which threaten the effectiveness of that agreed-upon-good are failures. This is not saying that that which has democratic support is right/moral, it's about allowing the existence of the institution we nominally support and allow for its uninterfered operation.

If, conversely, you don't believe that democracy is a good feature of government, there are bigger things to discuss than the impact of wealthy individuals on that process.

A lot of these discussions take this framing for granted.