Dollar is too much popular and it has taken a large period to make his place in world economy so bitcoin will also require a hard struggle and a longer period to make a place resemble to dollar, but do not think that it will replace or not.
The dollar is way too big, and the bitcoin is now way too small to do this, maybe in the future it got a chance but I honestly think its gonna be very difficult to do this even in the future.
yeah, currently dollar is too much bigger, because it has a large number of users, bitcoin can also become a bigger currency if more and more people start to come to adopt bitcoin and use it instead of all other currencies at least for their international use.