I don't think that the government will let the Bitcoin replace the dollar.
The Bitcoin will stay an crypto-currency but I think that it will be main stream over 15 years.
Yes, bitcoin will remain as crytocurrency and will be serving us for online affairs until the users of bitcoin will increase too more and I think it will take a lot of time and because the users of bitcoin are still very small and I think it will take such a long time when majority of the people of the world will be starting using bitcoin.
Here, we should not forget how new technologies evolve. They grow faster than we imagine. So anything could happen on the other hand. Like within one decade also, bitcoin may become as a currency preferred by all the human of earth. We cannot ignore that.