If US treasuries go steeply negative enough (and the spread remains as tight or gets tighter) then yes, banks will pay you interest, while you still pay princip

If US treasuries go steeply negative enough (and the spread remains as tight or gets tighter) then yes, banks will pay you interest, while you still pay principle. Lenders so far stay in the market because they actively trade the debt object even if they lose the business of earning interest. (With debt, the price of the contract increases as interest rates go down)