Last time I checked, America is still the best place for a millionaire to become a billionaire. Having arbitrary thresholds where people suddenly get taxed at obscene rates is, of course, foolish.
I agree this would be a challenge, but you can limit the percentage of a person's wealth that can be transferred out of the country. It won't be easy, but I still think taxing wealth (or a wealth tax combined with lighter income taxation) would be much more equitable than our current system.
So you're saying we get to keep all of the beautiful natural resources and culture of the country, but the handful of billionaires exploiting it and its people go elsewhere and take their shitty morals with them, leaving a country with reduced economic inequality?