AIUI, it's because the expensive part is not basically updating an internal database when money changes hands (no sending of BTC required), the expensive part is, to take WU as an example, operating half a million brick and mortar locations in 200+ countries.
Well what I'm saying is that it doesn't have to be the internal database. It can be a completely decentralised system. I send you $50 and you open an app and just walk up to the first money guy that you see on the street. Why do we need physical banks?
