The answers are yes, yes and yes.
It would be worth to replace banks in financial transactions of any type due to their costs. In a decentralised network there would be no central entity charging fees for that. The cost of a decentralised computation is negligible if compared to the costs imposed by financial institutions (even for trivial bank transfers, not to mention more complex financial tools). In addition, the decentralised nature of the Ethereum platform as of many other decentralised architectures is scalable. The core computing system of a single small bank will never scale to millions. That’s why it is a single small bank.