Will Bitcoin replace the USD? No. There is simply too much money involved for the world economy to let the USD fall. If the USD falls, we also fall with it. Bitcoin and fiat currency can perfectly work together to offer people the best of both worlds. Bitcoin will stay here for a very long time, the rest of the crypto will slowly fade away, and we'll forget about them as we only need to focus on Bitcoin.
Maybe there would be a new kind of crypto that would be issued by a central bank and backed by the government. The same would be just like fiat but in a digital form. I call it Fiatcoin. Now that might replace the dollar...
this is could be happens, looks russia gov, they interested with blockchain technology and want to create their own coin, but i think this is nice to see blockchain tech implement to the governments.
anyway, dollar can't be replace with other currency, even bitcoin. except the worse scenario will happens, war between US vs China