> And it's also true that most transactions, even for individuals, are not done in physical notes any more; they're done electronically (with debit cards or credit cards or wire transfers or whatever). So someone doesn't have to walk into the bank and withdraw their $100 in physical notes; they can do it any place, any time.

The difference between a credit card and a debit card is extremely relevant here. You can't run a bank with a credit card.