1) Banks can technically do that nowadays with one credit card, nevertheless, people are opening more than one cc from the same bank.
2) You will still need physical cards to spend at stores that don't support NFC.
i think we are going in circles...
1) today there are physical cards, so you can have many types of cards that a bank will target to different individuals i.e. someone who flies continental will like a co card, if i like hilton hotel i'll get hilton credit card etc. (i'm talking about normal people, not DDR's
if people wont use physical cards, than that wont be an option.
2)there is no reason a store cant support NFC processing like they do cc today