> This is true, but if everyone did this at once it would constitute a run on the bank and it would fall under its reserve ratio.Yes, it would. So what? That isn't what stops bank runs from happening. What stops them from happening (now) is that people believe that their deposits are "guaranteed" by the government, so there's no need to go rushing to the bank and withdraw notes every time the economy takes a downturn. Before that guarantee was in place, there were indeed bank runs, and for the very reason I gave.
> the amount of money in the banking system is far far more than physical currency in print.
Yes, that's definitely true. 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.