At a time where I see currencies moving away from cash to digital I think the reason is partly so that governments can control the flow of their currency more particularly for laundering. They can possibly gain more taxation from a cashless currency because cash cannot be fully traced. If i sell my car for cash to another individual the government dont get any revenue from that because it is not traceable.
So in a governments aim to control our money it will leave a big opening for bitcoin and other decentralised crypto to exist along with fiat and will allow people to "trade" anonymously with each other over a completely independent infrastructure.
Have you forgotten about blockchain technology and the first reason why there is bitcoin, bitcoin is a cryptocurrency that originally intended to avoid large costs only for transactions, here bitcoin transactions are carried out in P2P way where there is no third person as an intermediary. Although I feel your idea is not too bad to invite the government to work together using blockchain technology, and sometimes in some countries the government is seen in the eyes of the public and loses trust.