[imtringued]

The narrow bank restricts the Fed's ability to hawkishly raise the interest rate. It won't be able to get away with gross market Vs policy mismatches anymore. I mean think about it, the Fed makes an unintended policy error and raises the interest rate far above what banks can pay, everyone goes to the narrow banks. If the interest rate is too low nobody goes to the narrow bank. So the Fed essentially would have to perfectly choose the optimal interest rate which it probably can't do. It always overshoots or undershoots.

[TheOtherHobbes]

Which is ok, because that's just the market punishing the Fed for poor decisions.