Well, they will not develop something trustless, but they can make it decentralized. { Implementing nodes for example on all computers within all their bank branches } The only difference will be,

that it will be permissioned and private and they will control the development and what changes will be pushed on these clients/nodes. There are many Alt coins with some better features than

Bitcoin, but they do not have the main advantage... which in my opinion is the network affect and also the backing of a strong and secure network {hashing power provided by miners} that they do

not have to pay for. { In Bitcoin, miners give their resources and they get rewarded by the protocol } So for the users, this network is virtually free... but banks will have to pay for that resources.