I think that it won’t affect the main currency like USD or any other local currency of the respective country. The main reason is national currency can’t be carried away so easily which is representative of respective currency and it is also not replaceable because of traditional use of it. Just imagine how people or we ourself will react about it and that could be chaos for national economy too. Plus bitcoin can’t become main currency due to its limited number and insufficient reach to all over globe. So that’s simple why it can’t become main coin.