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 am agree that fiat currency always crash on the value. Fiat purchasing power always decrease because inflation and economic crisis always happen. The thing thats saving fiat money from crash is people always use it not matter the value always decreased.