I supposed one could come up with a Mark to Market scheme where I'm only taxed on the marginal gain from one tax period to the next...but that only slows the selloff of my core assets, doesn't it? But the key question is, would this go the other way...

...meaning if my XYZ stock dropped to $5 a share...would the government issue me a refund or a massive loss carryforward?

All legitimate questions, and valid concerns about whether taxes on unrealized income can practically be implemented.

But none of those objections invalidate the central argument - which is that if your shares increase from being worth $1K to being worth $10K, you have earned money on your investments. Your earnings are not realized taxable income as currently defined - but they are not zero.

Most of the largest fortunes in the U.S. have been accumulated this way. People generally don't become billionaires by having a billion dollars or more in taxable salaries over the course of their careers. They become billionaires through the appreciation of assets. Which means that Biden is correct, in that billionaires generally don't pay very much tax on the money they've earned. They do pay tax on the portion of the money they've earned that takes the form of realized income, but given how small a proportion that typically is, their overall tax contribution is very small as a proportion of their overall earnings.