The reason to tax billionaires is because no one should be tolerated hoarding that level of wealth.
The reason liberals love chanting "tax the rich!" is because it sounds good while posing zero threat to billionaires in any way whatsoever.
The only way to break the power of billionaires is through public infrastructure - the pharma bros can't make profit off medicines if it's being manufactured publicly, for instance.
The technocratic left and the libertarian left can spend all day arguing how public infrastructure should be run, but the one thing both understands is that the only correct course is public infrastructure.