Well cryptocurrencies, including bitcoin, will replace existing currencies eventually. Right now, there’s a mania that is getting in the way of that.
I would argue that distributed-blockchain-based currencies do have “external control” in the sense needed for a currency. The requirement is that something external is controlling the value and preventing forging, for example. With dollars, that’s anti-forgery laws, and the law that only the government can mint money. With gold, it’s chemistry and a limited amount on earth. With cryptocurrencies, it’s distributed blockchain.