I believe that is it absolutely impossible that btc will destroy the bank system.
Beside its unpredictability, how could btc ever reach every country in the world? Think about the third world and remember: no internet no bitcoin.
this is what i am trying to say too. for bitcoin to be accessed, people need internet, so automatically those who are i the remote are will not be able to use bitcoin. Second, even if bitcoin have replaced fiat money, there are several services of banks that cannot be replaced by bitcoin like storing important physical documents and many more.
The question I would then ask is what is the point of the physical money? There si no need for it as long as people can access the internet in some way to use the money. There is no need to use physical money, sop lets just change to digital currency.