It's been argued a lot that such a move would cause a good portion of American billionaires to just pack up and move to another country. Rich people are mobile in a way that poor people are not.
Although you've just made an argument for capital controls. We could call them "dollar export tariffs".
Of course collecting taxes internationally is difficult, but anyone wealthy enough to meet the criteria will probably need to visit the US at some point.
The US exit tax is a tax on your worldwide assets. The tax applies to all property that you own on the date of renunciation
That, and that the US is a tax haven for the super rich, so they have no reason to leave.
Where does their income come from? (e.g. investment growth, etc.)
Can't you basically tax their income, and quadruple that tax if they move abroad and want to move their money out of the U.S.?
(I'm guessing it's not easy, but I also guess the reason it's not done is because the ultra rich have so much influence on the politicians and tax code... not for whatever other logistical reasons might exist.)
The rich prosper precisely because they are in the US.
And, where will they go, really? There are considerable taxes in every country that billionaires would consider. And if they choose to cram into some small island tax haven in the Caribbean, the US can easily pressure that tax haven to do anything it wants.
Yet it's poor people that you see migrating most.