They would have two options: A) keep the euro notes in their vault, which only works if you deposit paper bills in the first place B) keep electronic deposits

They would have two options: A) keep the euro notes in their vault, which only works if you deposit paper bills in the first place B) keep electronic deposits in the ECB and pay interests to do so In either case if they give back the money to the clients when they ask for it how do you expect them to cover their operating costs (plus the interest they are charged by the central bank in case b)?