The only difference between digital currency and fiat currency is the ledger, which is easily applied to fiat, and the massive infrastructure required to make digital currency usable, which is a serious hindrance.
If fiat, and the infrastructure it supports, were to suddenly collapse, what good will your digital currency be? You can't access the Bitcoin network without an ISP.
Immediately what you are saying isn't true. Work is baked into crypto. You can't fictitiously print a Bitcoin or a Dogecoin but Trillions of USD can be popped out of thin air.
You are going to the absolute extreme and acting like all electricity and all routes to the Internet suddenly are destroyed. Like every node on Earth is instantly vaporized for all chains and the Internet is gone.
Yeah, things like that happen when pillars of society, like the fiat money system, collapse.
My greater point is that crypto by itself is not only unreliable, it's useless. You NEED electricity and an internet infrastructure for crypto to even work. I can fictitiously print all the Bitcoin and Doge I want, if nobody can check it against the ledger. Good currency needs to be usable in all situations.
If the electrical grid fully goes down globally there isn't going to be any money. It would just go back to trading food and weapons.
You are going to a far fetched extreme to where fiat currency and precious metals aren't going to have any value. It will all be about survival. Hunting and fishing or finding canned food.
The grid doesn't have to go down globally for your crypto to be useless, it just has to be down where you are. This is already becoming a real problem around the world as even fiat moves digital. When power goes out, stores become cash only, or they close. You can't go to an ATM, you can't withdraw from your bank, all you can do is argue with the store clerk, and ask someone to loan you money.
That's why crypto can't replace fiat. That's why credit can't even replace fiat, though it's been trying to for the last 40 years or so.
We do, technically, have the ability to get around this now, by distributing the internet across battery powered devices, but we aren't implementing it yet, and probably won't for quite some time, because it will cost the people with the server farms hundreds of billions of dollars a year in lost revenue.