>(selected for having acquired new wealth, as opposed to old family money that's more or less tied to the land)

Those are exactly the people we want to tax higher.

the nouveau riche generally actually do something to earn money, the trust-fund/inheritance class produces very little benefit but captures vast amounts of wealth.

Also, London has ever increasing real estate costs specifically because of all of the ultra rich foreign investors buying property in London. That seems to run counter to your claim that rich people will flee anywhere with high taxes.

Also to leave the country, you would have to pay an exit tax. You will also have to relinquish your US citizenship.

Those are two costs that i would bet the vast majority of the wealthy would not be willing to pay, no matter what the current tax climate is.