Because keeping private keys (by yourself) is troublesome, one of bank's services in the future will be keeping private keys for customers. They won't need to create their own currencies if they can convert enough of the population over using this system.
The customer visits bank and is provided a "bank account" which is a managed address on the bitcoin blockchain. The customer can still have personal addresses, but this address will be secured and guaranteed by the bank and have attached banking services.
I agree that a few people cannot take care their own private keys, but trusting banks to save copy of your private keys is terrible. you know, when banks save your fiat, they lend it to the other people and get interest. I think they would do the same thing. and that's against bitcoin rules.