It is not possible for bitcoin to replace the dollar when there is more than 80% of the world population that has no idea about bitcoin. It will take decades for all people to know about bitcoin and believing in it is a far away thing.
What we should be asking is that can Bitcoin be equivalent to dollars, we are aware that dollars is a fiat currency and fiat currency have a lot of users before now, and is well known that each country has it own currency, all the fiat currencies using for all the nations is as ancient like century, so Bitcoin is a newborn currency that have not gotten up to fifty years of adoption, but we are comparing it with dollars that has been in existence before so many of us, Bitcoin has occupy some space before it will use to compare to dollars and replacement of dollars, even till this length Bitcoin has not been accepted with all the transactions of exchange of goods and services, some companies like Amazon rejects payment with bitcoin but accepts dollar fiat currency, so the difference is there between dollars and Bitcoin.