Governments also have a number of additional regulatory functions in addition to raising funds. Things like seizing the proceeds of crime, preventing fraud etc. Monetary policy is more than just a money raising exercise. There can be good reasons for devaluing a currency.

Think for a moment about the implications of a world with a single currency mediated through a block-chain predicated on the idea than no-one can ever get more than 50% of the processing capacity. Think about Angola, China, Sweden and the US sharing that currency