The reason why banks will always be against cryptocurrencies is that, since btc in its nature is a decentralized cryptocurrency and that they will not get anything from it when they allow all people to accept it.
I agree with what you have mentioned and bitcoins decentralized nature is the main fact that banks will never work in favor for bitcoin or its existence because there is no possible way for them to gain anything from it except if they provide services for its users to convert bitcoin into fiat or the other way around but it is not profitable at all and if most people starts to deposit there money in bitcoin wallets instead of using banks they will collapse so banks were against with bitcoin from the beginning