I dont think so bitcoins will destroy the fiat currency although 50% of people in whole world will use bitcoins fiat is still main currency even after 30years from now.
For me it would be used by most of the people who knew a lot of bitcoin and there would be an upcoming changes in the future that everyone will use bitcoin as a method of payment transactions but it wouldn't kill the fiat currency. Bitcoin needs the fiat currency in order to purchase local goods because i don't think so local goods can be link to PC transaction specially on those remote areas around the world and if that happens its more costly for the business holders to adopt this kind of cryptocurrency worldwide.