"Most European countries have steep inheritance taxes to prevent going back to oligarchy."

Which has the problem of course, that the really rich have ways of not beeing affected by those laws too much. Which is not too hard, when the tax-laws are ridiciously complicated with many exceptions for good connected lawyers and many countries on this planet who welcome any people and money as long as they get a small share.

That's unfortunately true, but doesn't mean we should just give up and let society slide back to aristocracy & serfs. It's perfectly possible to elect representatives that deal with various tax avoidance schemes (be it corporate or personal).