I don't think that anyone ever thought that paying the bank to hold your money was a theoretical impossibility -- paid checking accounts are not a new thing. What is supposed to be 'impossible' is for bank loans have a negative interest rate -- if the bank pays you to borrow money. Of course, even that was/is only 'impossible' with certain exceptions (specifically, deflation is bad for lenders; but they try to predict deflation, and try not to loan at a negative real rate).