There is simply no way a rational businessman would take all this risk and pay such high rates unless he was making a significant profit. And the first thing a rational businessman would do when making a significant profit is pay off his absurdly-high interest debts. And if the risk is high and he nevertheless had the funds to cover losses, the most rational thing to do with those funds would be to loan them and thereby get rid of his high-interest debt. No rational person would have $50,000 in a bank account and still borrow the equivalent of $50,000 in BTC at exorbitant rates.
Here's where you fail..
First, if a rational businessman is taking on a bunch of risk and paying high rates, he must not have much capital of his own or else he wouldn't enter into such an arrangement in the first place.
Second, if a rational businessman is taking a large risk and has to pay high rates, he probably has a high gross return, but when factoring in the high rates he is paying, the net is probably much smaller.
Third, which relates somewhat to the first, is that a rational businessman knows that things come up. Unexpected things. When those unexpected things come up, typically, they're costly, meaning they need capital quickly. When borrowing, you can't force people to lend to you. A rational business man knows having a reserve of profit in those instances helps to fill those immediate need situations.
TL;DR In short, there are times where it makes sense to keep cash on hand vs paying off outstanding debt.
EDIT: I forgot to tie this back to PH, since that's who seems to be who everyone has a hardon for lately.. He's not accepting any more deposits and honoring all outstanding obligations. I'd say that's the largest evidence of 'not ponzi'. I've not seen any of his customers (borrowers or depositors) dissatisfied, so unless you want to call them idiots too, I'd say things are working just fine.