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.
It's more about local use than international use because if local economies start paying people in Bitcoin and real estate owners and grocery stores start accepting Bitcoin then it will be a lot easier. Obviously at the start those business will need to sell the Bitcoin for fiat but eventually Bitcoin will sustain itself and then people will only see and use Bitcoin.