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.
Bitcoin is just a good alternative payment and bitcoin is less hassle than the other payment proccessor..
dollar is our main money and it is impossible that bitcoin will replace bitcoin. how about if we have mo internet so bitcoin is useless and usd will be still need to use..