A question arises: Would taxes still be applicable if a time comes when exchanging bitcoin for fiat is no longer necessary?
I can't directly answer your question simply because I really don't see something like that actually happening. Or maybe it could happen in some small countries but I can't even image countries like the US, the European Union, Japan or China using bitcoin like their first currency, they would have no control over it, they couldn't print more, it's something pretty much impossible.