Why would you post such an idiotic article? It is non-sensical on its face. The article itself states the following:
Their wealth derives from the skyrocketing value of their assets, like stock and property. Those gains are not defined by U.S. laws as taxable income unless and until the billionaires sell.
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The federal income tax in the U.S. (like most countries) taxes annual income, including realized gain on assets sold within the past year, but not unrealized gain on any increase in wealth. So, the discussion is idiotic.
It is just an argument for a wealth tax using a self-serving and inaccurate analysis of U.S. income tax law. If your house doubles in value over ten years, should you have to pay a wealth tax on that increase in value even if you never sell it?