I think the US government is developing its own cryptocurrency right now, which might become a national currency
National currency will be accepted by a country when there are large number of people follow the same currency and I think bitcoin is not still at that stage but it is more possible that bitcoin will cross this target and will become national currency.
The Btc problem today is not the scarcity of currency but the high rates and the overhead in its transactions so that a country adopts it as currency would have to solve all those problems that it has today. On the other hand the difficulty of obtaining btc would not be so much because the currency works quietly fractioned, then would give to use it in the day normally, besides facilitating the payment of accounts inside and outside the country.
You have to know liquidity, fast system and cheap fee all the requirement for a national currency. No one wants to spend more than 10 dollars for every transaction through Bitcoin though just need to spend few dollars for payment. Beside that, the current capacity of Bitcoin system isn't enough to becomes a national currency
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