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 don't agree with this because first of all, keeping private keys by yourself is one of the main purposes of bitcoin! to be your own bank without any central place to rely on! so if you want to keep private keys at banks then just keep fiat in banks, why bother (and we are talking about future, and price won't be as volatile now)
also keeping private keys safe on your own is easy as cake.
either pay for hardware wallet and keep safe, or other many cold storage methods for advanced users.
p.s. making a bitcoin bank account like what coinbase does and customers don't have private keys is different from holding private keys.