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.