People will continue using fiat even if there is a chance that cryptocurrencies will become the standard currencies in the future. Not all people have a knowledge of computer others have no idea how bitcoin works especially in the rural areas and to those who are illiterate people. They will continue using money because for them it is the easiest way, consider that bitcoin is more technical than paper money.
And what do you think about long - term? like more than 50 years.? I think it can be, cause in future most of people would have enough knowledge to use computer technologies...but also, maybe, it will be a lot of differents from todays technologies, so our generation will not know about how to use future technologies )