...the major role of banks will still be the same as nowadays: give out loans and do investment banking.
I would half agree with what Chrithu wrote above: banks will still give out loans but that is
not their primary role nowadays.
The primary role of banks in a 100% bitcoin economy would be to assess the creditworthiness of prospective borrowers, lend depositor funds to them at rate of interest commensurate with risk, and profit from the difference between the interest rate charged to borrowers and the interest rate paid to depositors.