Having a nation state to back up your currency isn't necessary. Back in the early 19th century the United States depended primarily on private banks issuing currency (banknotes) on the basis of deposits (hard currency--metal, usually in the form of gold or silver coins). There were problems, like they often over-issued, putting out more banknotes than they should have relative to the deposits. And the banknotes circulated well away from the banks that initially issued them--making it difficult to know if the bank was still solvent.
But it worked. At the macro level it was because people needed some kind of currency to keep the economy rolling and there just wasn't enough coinage to support it. At the micro level, it was all about a kind of group acceptance of the banknotes as being legitimate. Would you accept a particular banknote as payment for something? Well, if you were reasonably certain that the next person down the line would accept it from you, then yes.
Coming back to today, the question to ask is, "Does BitCoin have enough volume of currency in circulation to meet the demand?" Or to rephrase it, "Is the BitCoin money supply sufficient to meet the needs of this particular part of the economy?" If the answer is yes, then there really isn't a reason for other, similar currencies to be created. But if the answer is no, then other currencies might be viable.