Maybe it is less about the tax revenue and more about preventing power imbalance. Why is it good to have so much power concentrated in the hands of one person?
They are really not better than government because these people are the one who lobbies government to vest their selfish desires.
You should check out the shenanigans that the US government gets away with. Whatever bad things Musk, Gates and crew is small fry compared to the government.
A side effect of a wealth tax is that instead of wealthy people building wealth that's just going to be taxed away, it would incentivize funding charitable foundations like the Gates Foundation. Bill Gates has done a lot of good with his foundation, but that doesn't meant that one person should control that much money.