Well, for Bob's bank to owe $50 more to Bob they need to be compensated so Alice's bank now owes $50 to Bob's bank. In other words, Bob's bank must trust Alice'

Well, for Bob's bank to owe $50 more to Bob they need to be compensated so Alice's bank now owes $50 to Bob's bank. In other words, Bob's bank must trust Alice's shitty bank. Unlike the dollar, Bitcoin is not the transfer of debt.