Ah, but when they transfer that money, it'll be to some customer of some bank whose loan customers will also be transferring money in the other direction. This

Ah, but when they transfer that money, it'll be to some customer of some bank whose loan customers will also be transferring money in the other direction. This means, in practice, that the main constraint on a bank's ability to lend money is how willing all the other banks are to lend money at the time. The Bank of England paper linked earlier in the thread explains why this has undesirable economic implications that are missed by the incorrect money multiplier model.