I think that is impossible because we still need the bank as a medium to withdraw funds from bitcoin wallet against the legal currency in our own country. However, if bitcoin is legalized all over the world, the bank may still exist but bitcoin can enter as a recognized currency in the bank and can be used for direct transactions.
Why do you need bank as a medium? You can convert Bitcoin to fiat cash online, using any of the exchanges. Or you can use pre-paid debit cards and load them with BTC. Once this step is complete, you can either use the funds for shopping, or you can withdraw the money using any of the ATMs. Banks are not needed at any point of time.
Why not? In my opinion, the banking system still has an important role for daily. If we have not the banks, we can not exchange our money. Of course, some people will say when Bitcoin become legal, we do not need to have bank in that time. But remember, on the world, there are have many people still do not know use Bitcoin, even Visa/Mastercard as my parent.
Never, it is not possible that bitcoin when legalize then it will disappear bank. everything has his own role in the world just like that bank have a big role in our daily life without bank there are many works which are not possible. but it is true that when bitcoin legalize then it will down the value of banks up to some extent.