I have to say it is quite possible, if the bitcoin on legal by the government then the bank transaction will drop, everyone will switch to bitcoin because proven better than other payment systems including banks.
Certainly, it won't happen since bitcoin can be only be used for online transaction which means if it would replace banks, not all poeple can adppt it since internet connection isn't available to all place in which is bitcoin replace banks, those people who doesn't have internetconnection would not be able to withdraw their money.