Probably bitcoin will never replace dollar, IMO at most it will become widely used, similar to euro in Europe.
Certainly, for sure it can or it will be used widely in the world after a few years, though it is already used worldwide but it will get more popular with time but as you said it wont be able to replace a currency like dollar which is used in a large extent in the world.
Nevertheless, popularity isn't the only one thing which bitcoin requires to be a currency like a dollar. With that BTC also has to be currency that used by states. Since this use is supposed to be total, BTC also ought become currency for taxes.
However, that'll kill BTC. BTC is special just because it exists without states and taxes. And BTC replaced USD will became a new USD.
When BTC fights
monster USD, it becomes
monster USD (allusion to Nietzsche).