When the cash flow from the assets exceeds interest expense, you've cashed out the assets without incurring tax on your appreciated position and you can afford to pay the interest. As for principal, debt is largely not paid back these days, especially large bespoke debt secured by liquid and well-defined assets. The debt holders (lenders) get paid back after death of the borrower or they continue rolling the position and collecting their return (interest income). The only question in the lender's mind is how much leverage to grant on the underlying assets, e.g. blue chip stocks, and what to do in a liquidity crunch when rolling.