first of all, cryptocurrencies will never eliminate fiat currencies, cryptocurrencies will only replace the dominating role of fiat currencies,  At present the majority of the world's population is filled with people aged over 40-60 years (most of whom are not familiar with the technology) in a few decades, people over the age will die and will be replaced by young people (who master technology).  as time goes by the human need for efficiency is increasing and the cryptocurrency will become ''Cash in the future".