If blockchains can reduce the non-fractional funds that banks have to fully reserve and keep at other various banks to facilitate and settle transfers by even a few % the savings might be even higher, especially once you actually start getting ROI on that money that you can invest now.
That's not even taking into account the ransom banks pay for correspondent / agency banking and for SWIFT and the likes.
As well as the cost of liability of invalid/bad transfers, and banks oddly enough still "lose" money all the time, some of it is later recovered but virtually every time banks do account consolidation the books don't add up.