Banks can use blockchain technology but it is never going to be in decentralized manner so it cannot be compared with the crypto adoption if banks starts doing transactions with blockchain.Either bank employees may not let it happen sice if the blockchain was introduced then most of the employment in banks will get vanished.
The development era is more advanced and developing and technology will develop, so will the economic development will also follow it and one of them with crypto currency is an economic system that uses technological developments and banks will also experience system development as well.
Except banks create their own cryptocurrency before they can agree to follow the technology of Blockchain, because naturally, the reason why banks are now thinking of Blockchain is because they have realized that they dont have to engage in cryptocurrency before they can also get the benefit of Blockchain, but as bitcoin is already existing as a product of Blockchain, they are being forced to actually go for it too since they know that they cannot even do much again bitcoin.
I believe that in future, as banks actually develops their own Blockchain, we may be seeing some of the cryptocurrency being created by them which I think would not be very much okay as this will create unwarranted saturation for the cryptocurrency market.