I don't believe the banks can be completely wiped out in a moment but I foresee the banks rather investing in bitcoins and gradually replacing them (their fiat) with bitcoins in the future.
They don't need to invest much in Bitcoin. They issue loans nominally in Bitcoin and hand out checkbooks. They don't have to hold the full amount of the checking accounts in Bitcoins, just a fractional reserve. And their loans get paid back in real Bitcoins.
Pretty soon they have a lot of Bitcoins.
Yes that is called fractional reserve banking and it is one of the biggest tricks of bankers that is why we must avoid the creation of bitcoin banks because that is what they will do at the first opportunity, we are our own banks and we must have control of our private keys and no one else.