This would only stop borrowing via loans that are collateralized by stock assets. These banks would presumably still offer uncollateralized loans to these folks

This would only stop borrowing via loans that are collateralized by stock assets. These banks would presumably still offer uncollateralized loans to these folks (you have $X billion dollars, I'm confident you'll be able to pay off a loan for 2% of that, here you go, no collateral needed). Then, the end effect will be slightly riskier loans for the banks, and slightly higher interest rates for the billionaires. And if you set the limit at $500k, you're going to block rich, but not super rich folks from taking out loans to start new businesses, etc. To me it seems this would hurt more than help.