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.
I would not say everyone will switch to bitcoin. People have their own perception when it comes to bitcoin. Others may say that bitcoin is good, some may say it's bad, or others will not care at all. And think about this, not all people uses the bank (that includes me) so how come that everyone will use bitcoin.