Most of the revenue used to finance the Built Bigger Blowout comes from taxing years if not decades of unrealized appreciation (paper profits) of the Evil Rich. After that first hit, as you say, their effective basis would be whatever their assets were worth at the end of (say) this year, so only future incremental gains in the market, net of losses relative to the taxed EOY mark, would result in additional taxes due. So mostly it’s about stealing some 10-15% of their net worth from the billionaires right now (who often hold highly appreciated stock in companies they created ala Amazon or Tesla). Then, as an afterthought, they have this ongoing MTM tax regime for the Rich instead of letting them defer unrealized gains like everyone else.
It’s also very disingenuous of Biden to say he’s going to make Billionaires pay at least 20% on their income, when what he’s taxing is definitely not “income” under the law. They’re not “getting away with” anything more than anyone else right now - paper profits aren’t income until or unless this law passes.