i think that nothing is impossible. The problem is only time, and time here will be counted as a millennium or more and I do not think we'll be alive to see that happen. But now I can be for sure that in the near future, bitcoin will replace the dollar because eventually everyone will use bitcoin.
That would that all people will use online transaction, it's the only way I guess which I think would not be possible.
Every country has different standings in the economy and that is why we have poor countries because they lack infrastructures which will make their economy not to progress faster, with bitcoin I guess it's not their primary concern yet, they have big problem with education and all they know is the conventional way and they will look for money but working all day, they do not need this technology so we can exempt.
When bitcoin will replace dollar, that means it would be easy for small value currency to be replace, I just don't see it happening.