(1) Strongly suspect based on what? There's 0 precedent for the scenario you suggested, but plenty of examples of right-wingers using this idea to scaremonger.
(2) Where are you actually going to go? European countries already take higher taxes, and most other countries have higher inequality and lower general standards of living than the US. I don't doubt that some of the wealthy consider themselves "global citizens", but the majority of people aren't going to leave their home country over marginally higher taxes - especially at today's pitifully low rates.
(3) Most of the upper middle class people I know will stop being hysterical about paying higher taxes after a moment, and recognize that a billion is 1,000x more than a million, and 10,000x more than 100K. American's don't hate wealth, they hate aristocrats, which is what the billionaire class has become. The same logic does not apply to people with "mere" eight-figure wealth.
2. If I understand correctly, this proposal is not an income tax, it is a wealth tax. I know of no European country that has such a thing.
3. See 1.
I guess that's somewhat true, yet the upper middle class only got stronger and stronger, which seems strange if they were all fleeing the jurisdiction because of high tax rates. It seems much stronger evidence that it's not a real threat that should concern the country - most high income Americans are reasonable and understand that we need to pay to maintain all this modern stuff we have, and hopefully improve upon it. It wouldn't seem to be a major concern if a small, selfish minority decided to pay the exit tax and go somewhere else.
Netherlands has a wealth tax system, just for reference. I agree with your 1. point
You should read history books more often.
I'd be confident in calling out the rich on this bluff.
(But, as I said on another comment, his theoretical net worth isn’t real money, it’s how much other people reckon they could extract from the companies he controls).