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 think it will be more years before that happen, Fiat and Crypto currency are almost the same in nature of ups and downs, There are also a crashed in crypto currency, There is no perfect system,We just need to be wised to consider the one that has lowest risk.