Some of this people that have spoken against fiat currency are huge investors in cryptocurrency. The crash of Fiat's has always happened to various country currency. Just as experienced in crypto. This is mainly due to market players interest. All Fiats cannot crash at the same time, there willnbe a rise and fall to ensure profitability of key investors.
I don't think it is possible for now for fiat currency to be crushed, Most of the countries in the world is using it as their means of payment, Only few persons and countries are using crypto currency , I think it will take more time for the crypto currency to be considered by some people or the governments of every country itself.