The thing about billionaires is that it's almost entirely paper wealth.

The real thing they have is a collective agreement that allows them to have the power to direct a bunch of capital toward production. If they ever just decided to cash in their chips and buy a dragon hoard of gold instead, the paper wealth would vaporize really quickly.

The problem that really exists here is that this small group of individuals has too much power. The feudal system existed for a long time, there's arguably a tendency to return to it, and this is the thing that truly makes people uneasy about the concentration of power outside democratic processes. How do we get rid of the despots? At least with democratic process there's a way to vote them out, their terms expire, etc. With hoarded wealth, and especially (as we see with cases like the Waltons and now the Ellisons) generational mega-wealth, we have zombie feudalism clawing its way out of the grave.

I don't think there's a problem with someone earning the trust of a bunch of investors and being granted a lot of power to direct resources in their lifetime, but the intergenerational transfer of unearned power is a place where a crackdown is certainly warranted.