It is Clear that bitcoin will be the only Currency for payments
What is the evidence to this, we cannot conclude yet, bitcoin can be the mainstream digital currency but doesn't equally means that the world will only have it as a means of payment, fiat may also exist along and the future ahead will determine which one will the people value more with higher priority.
There are absolutely no clues to that at all. Some people are just way too overboard with bitcoin world and I understand how excited they could get about it but we are quite certain that bitcoin will stay an alternative. It could very well be something that turns into legal tender one day in many nations, even then it won't be all nations but just some of them, but I believe that the best thing we could do at the moment would be just accepting it the way it is at the moment.
I believe that bitcoin will not become the single currency that everyone uses, of course it can be used and accepted in many more places and by many more people but that is just the adoption rate going up, it doesn't mean that we are going to do fine.