And yet, study after study shows that within two or three generations, the children, grand-children, and great-grand-children of the ultra-rich by and large ooze their way to the middle class. Dilution by generation, dilution of the work ethic and drive that their ancestor used to amass a fortune, a "spend spend spend!" mentality instead of a "save & invest!" mentality... the money ends up in the general economy, generating more tax receipts as it goes.
For the oligarchs and oligarch-connected, even the estate tax doesn't work -- they have set up family foundations and trusts decades in advance. Can anyone say "Kennedy?" Rose Kennedy, who lived in MA all her life and was uber-rich, was found, upon her death, to actually be a legal resident of Florida. Not subject to Mass. estate tax.
A cartoon in the New Yorker some years ago showed a couple of guys in suits in a high-rise office. One says to the other, "These new regulations are going to fundamentally alter the way we get around them."
And that's about it for the oligarchs and their nepots.