This is the same thing as going to every homeowner in the country and saying, "The value of the house you bought in 2000 has doubled. Therefore, you owe tax on the 'income' you've received". That's asinine.

But it's also asinine to pretend that someone like Warren Buffett has had "only" a few hundred million dollars in earnings in his lifetime. He went from a negligible amount of assets to having a net worth of $100 billion. He must have earned a lot more money over the last seventy years than the $10-20 million per year he declares in taxable income - you can't go from $0 to $100 billion earning that "little."

Unrealized appreciation of assets is not "income" within the meaning of the tax code. But neither is it nothing. If one person bought $10,000 worth of shares in Apple in 2000, and another bought $10,000 shares of IBM at the same time, they'd have about $2,000,000 and $20,000, respectively. One person has earned nearly two million dollars on their investment, and the other has earned only ten thousand, and it's asinine to pretend that they've both "earned" exactly the same - zero - over that time frame.

For lots of reasons, we do (and perhaps should) exclude unrealized asset gains from a formal definition of income subject to taxation - but an informal understanding of how much of billionaire's "earnings" are actually paid in taxes doesn't require us to pretend that massive increases in personal wealth in unrealized holdings don't exist.