To be fair this is about wealth, not income. Globally there are countless taxes on wealth, including unrealized gains. Those also exist, IIRC, in every US State and Territory. Most fo them are real property taxes (that one is, I think, ubiquitous). There are also other forms of indirect wealth taxes including various types of sales taxes, property transfer taxes and so on.
That some of us seem to view a direct tax on unrealized gains as an affront to democracies not surprising, but it is disappointing. Bluntly, I am not thrilled about the idea, it would cost me an inconvenient sum. That may well be the point. What to do with wealthy people who have structured their lives to avoid undesired taxation? Around the world there are many solutions.
Regardless of my personal aversion, such a levy is obviously legal and can be done. Nobody will change their lives if they pay a similar levy to that proposed for corporations that avoid all taxation currently. Bizarrely almost everyone who falls likely in the bullseye do not now pay any AMT, precisely because of having minimal 'income'.
Until those who are in this category are paying a similar total percentage tax burden equal to any random member fo support staff, there is every reason for government to exact a little bit of wealth tax.
Elon Musk will not alter his stellar performance if he must pay some tax. Neither will Jeff Bezos, Bill Gates or Warren Buffet. Like it or not their wealth is a reflection of their accomplishments, not the motivation for them to excel.
Dispprove if you like, but not on the basis of political posturing. That is rather disingenuous.