I have yet to see it being pointed out that none of the encryption in use for these cryptocurrencies will necessarily remain extremely hard to break for long. A good question is: what if bitcoin or something alike does effectively gain traction and one day a math breakthrough (or a computational tech one) make the crypto weak? How much wealth will be destroyed then? Who would you turn to for help? Our system is not perfect, but taking central banks out of the loop is not a way to improve it. It would be much more useful to find ways to push down financial transaction and currency exchange costs.