> demurrage and land value taxes and it's game over

I don’t think that actually solves the problem. For example, if a citizens entire net worth is stored on the New York Stock Exchange and you are not the USA how do you plan to tax the individual? The NYSE is bound by the laws of the USA — which has strong property laws — not the other country.

> they get replaced by those who are willing to fill the void they left behind

If only a single country has a wealth tax, there are going to be no wealthy people who come in to replace the void when they have a choice of many other countries that do not have a wealth tax. You could possibly argue that a country is better without billionaires anyways as they don’t pay into the tax system, but I believe that if a country implements a wealth tax without coordination with the rest of the world they will simply no longer have wealth.