It's not like the money is just sitting, liquid in a vault like Scrooge McDuck.
Idea: tax loans taken out using assets as collateral at regular income tax rates. After all, that money gets used like regular income (living expenses).
The taxed amount can then be added to the basis when the asset is sold. It would be like reverse of depreciation calculations.
Set an asset and loan value floor so it only affects people with assets $10M+.
After all, regular people pay taxes on annuities, which are similar in structure.
Disclaimer: IANA-Accountant, but I am a taxpayer who tries to legally minimize my taxes.
In the context of home ownership, a loan using an asset as collateral translates to a home-equity loan or reverse mortgage. If you want to protect ordinary home-buyers, set an asset value floor of say $20M.
However, I think most share "pledging" [1] by the uber-wealthy is done using company stock as collateral, so you could restrict the tax further by having it apply only to loans taken against stock holdings over some similarly high value floor.
1. https://aaahq.org/portals/0/documents/meetings/2024/ATA/Pape...