So, when you hand over your pile of failing fiat money, what exactly do you get in return?
When someone says things like "failing fiat money" I realize they're posturing, not discussing. But I'll answer anyhow for the people who have open minds.
What you get with a fiat currency is stability. If I sell you a ton of hay, the $250 or whatever I get from that is still going to be worth about a ton of hay when I use it next month to buy diesel fuel. If it wasn't I'd have to try to buy diesel with hay. That's a problem because the diesel guy doesn't want hay, you do. The whole purpose of money is so I don't have to try to talk the diesel guy into taking hay. Instead I can give him something with a universal agreed upon value that he can use to buy anything he needs.
Inflation and deflation can change currency values, but that's generally bad and to be avoided. It causes a lot of problems. If you think the currency is going to appreciate rapidly, you're not going to want to give me $250 for a ton of hay. You may hold off buying hay, which is bad for me (and the economy if a lot of people do it). Or you may try to buy it for less than $250. Now we're not only negotiating the price of hay, we're also negotiating on our feelings about the future of the currency we're using to trade hay. That's bad for commerce.
Cryptocurrency's value fluctuates wildly compared to any real currency. That makes it terrible as a currency. "Stable coins" try to address that by (usually) pegging the value to the dollar. So now you're using something that's got a stable value, but it gets rid of many of the supposed benefits of not being fiat currency, and still keeps the drawbacks of crypto (lack of privacy and incredibly high transaction costs).
Anyhow, go buy some bitcoin or doge or whatever and then try to buy hay or diesel with them. You can't. It's clear that they're investments (of questionable quality) not currency.