How do you do that? With what do you pay back the loan? With a new one?

counter-party risk + collateralize risk is still risk. 1. You are risking a larger amount of bitcoin with a counter-party so that you have wider margins; flash crashes do exist. 2. If you don't have fiat inflows you cannot settle your debts and you end up paying in bitcoin anyways, potentially at a lower price + interest. Presenting this as a catch-all solution might be dangerous, different situations call for different solutions.