I think it'd be popular among citizens who make under Y/year. You can make Y arbitrarily high, like $10m/year, and make X quite reasonable, like 30%.

Stepping back, to look at whether such a tax would be useful.

The chief risk that thinktanky-people raise is that there are many desirable countries to live in, which don't levy taxes as such. So you'd get a lot of assets fleeing $TAXING_COUNTRY, which, the argument goes, would have a deleterious impact on people inside that country.

In practice, most income of these people is not circulated in the American economy, but immediately invested into "stationary" assets, such as securities issued by US Corporations or the Treasury. Regardless of where these people live or hold citizenship, much of that stationary income will continue to be invested in the US, where real growth rates have historically been sustainably-high.

(a simultaneous tax on US-held-wealth, not income, would throw a big wrench into this argument!)

In addition, I doubt many people would actually give up their American citizenship to avoid this tax (simply having a residence abroad would not be enough to escape this tax, if you're a citizen, since our country taxes citizens living abroad).

IMO, most of the people who would give up citizenship to avoid a mandatory income tax, would have already given up (or avoided obtaining) their American citizenship anyways, because of the tax implications that exist today.

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This talk of emigration reminds me of a fun thought experiment.

I remember reading some argument whose thesis was, roughly, "one reason the US is able to accrue so much debt, well past the reasonable point of leverage for the average country, is because the US is the only nation with sufficiently-international tax-collecting, finance-monitoring, and military potential to effectively implement a wealth tax".

From the buyer's perspective, Country X can buy our debt, essentially as a securitization of a future wealth tax against the US. Furthermore, this effectively doubles as insurance against wealthy emigres leaving your country, since most of them take their assets to the US when they go! This means that the likelihood of US Debt being paid back is extremely high, even past the normal metrics of over-leverage (debt as a % of GDP).

To avoid a hypothetical tax on wealth generated in the US since the 1940s, you'd need to move yourself and your assets to a location outside the combined military+financial influence of the USA, that is willing to take your now-contraband, non-USD assets. Any USD assets held by most banks would be de facto seized.

Presently, maybe North Korea and Iran fit this bill. Russia, to a lesser extent, but this is a roll of the dice. Not the best places to peg a long-term future on. It's possible to imagine China (and the countries under its sphere of Belt+Road influence) fitting into this category one day, which would be interesting and probably invoke a war.

One interesting feature of this lens, is that the US shouldn't ever need to implement a wealth tax to justify the borrowing ability, the US simply needs to make it clear that such a tax would be possible to levy, if absolutely necessary.

Another interesting implication is that, if the US should ever lose its power to levy this tax (due to a sharp decline in military, economic, or financial policy strength), the US's ability to borrow way above our annual-GDP could evaporate overnight: causing all kinds of mayhem, probably.