The problem is : if you want a stable currency you need a money supply that grow as much as `inflation + growth + the slowdown of the money flow`, if you don't have that your economy collapses. All of this is changing over time, then you need to adjust the supply of money dynamically. And unless someone finds a way to do this automatically, you'll need a pilot in charge of this. This is currently the job of the director of the central bank, it can also be the president of the country, the parliament, or anyone but you need someone (or some cleverly designed distributed mechanism that doesn't exist yet). If you don't address this, cryptocurrencies aren't better than gold money, which historically proved inefficient in an industrialized society.