Mortgage and auto loans are usually cash secured, so there are no gains to realize. This tax treatment could (I would say should) only apply to e.g. shares on t

Mortgage and auto loans are usually cash secured, so there are no gains to realize. This tax treatment could (I would say should) only apply to e.g. shares on the difference between the cost basis and amount used to collateralize the loan. One quirk of that is that regular Joe could then get a loan against their cost basis in their portfolio and still have no taxes. E.g. they bought 10shares of $X at $100 and now it’s $200. A loan secured against those shares would not be taxable up to $1000 and the portion between $1000 and $2000 would be taxable. This still stops mega billionaires from never paying taxes because the cost bases on their shares are usually very low.