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.
Through this years many users respond that fiat will be crashed and be useless even in the future. Inflation happens in the economic system and exactly it rise and fall but I dont think fiat currency would be crashed.