I didn't see anyone think of a way that would resist:
THE WORST THAT CAN HAPPEN
Computer Technology Corruption such as online or hardware hacks that destroy or are watching your key/s when you are transacting are a threat. Another possibility I find in my research is that there could be an EMP bomb over the skies that fries all electronics, there are solar flares all of the time, and one could randomly hit earth frying all electronics. There are bad government actors that can access the cell and wireless towers which could be turned up to fry electronics. Wars are a common threat in civilization now and throughout history because of the depraved getting rich quick by hoarding all of the money information, laws, deeds, beliefs, rules, disinformation, and political/corporate/bank/government/cult/pseudoscience deception. Blochchains are meant to prevent those things simply by keep records straight and balanced truthfully, but the above threats still loom more than ever in these days of unconventional warfare or science wars which use all forms of phenomena in massive, elusive ways without using a single gun or bomb like before.
Also you can break computers. They can be stolen. They can be lost or whatever. People lose their things. That's the fact.
Essentially your bank is your computer, someone else's computer, and it is electricity and a combination of data for the most part. That means that you have to make sure that it is never able to be compromised by any elements including others, yours errors, or elements otherwise. If you lose a little money, hey, everyone does that. I've lost twenties, change, and a few hundred dollars once. I literally through my work checks in the demolition heap at a job site with my lunch garbage when I was 19.
PAPER WALLETS
I know people want to store on paper. The paper wallet trick. It's a good idea, but paper and ink disintegrate most easily. You can also lose it by dropping it, whatever it is in gets stolen, or you forget where you put it, or you move, or you anything, and it's left behind etc etc etc.
METAL WALLETS OR METAL KEYS
I suggest what is better than paper for long termers that are hardcore preppers or you know who you are...engrave the keys into metal. You could just scratch it on metal where there is no one that will ever do anything anyway which is always covered up and it's not possible anyone will look there. It could be a shed, a building, in a wall, on pipes under sinks, in the attic, in the basement works, in the utility room works, pull the washer and dryer out or something tack it on there, pull a panel off of the car, weld it onto big rig equipment machinery or metal supplies or tools or something like that or a beam inside of a building etc.
You can take a little dremel with a cord or with a battery or some kind of drill like that, and just go carve it in the metal, rock, or whatever. These tools and the supplies are cheap on ebay, craigslist, etc etc etc.
Things can happen though. Cars can be pulled away. Cars can be stolen and wrecked. The house can burn down, and the washer and dryer or whatever could be so badly warped that the scratch on engraving and even drill engraving would be unrecognizable if it's even accessible in the first place before the bulldozers come push up the junk, and haul it all away. If it was in the basement or something like that, a burnt house would cave right into that hole you call a basement. Good luck getting it out of that.
Scratch on a piece of metal may wear away over time like say a decade or multiple decades.
Perhaps getting a grouter and carving out of the most inconspicuous place on a concrete foundation or something like that would work.
Perhaps having keys engraved in metal strips by an engraving machine, and then bury the keys where you can't forget. Use significant landmarks which can't be bulldozed over or lost to mudslides, flood erosion etc. Think ahead, and think what the world does. If it can do it, it probably will happen to it. Just think like that. You are number one with a bullet. That lifetime of yours and that key could mean everything someday, so think about it hard. Think about it again. Think about another time. And think about it again. I used to be an arborist that climbed and rigged all of the trees. Always quadruple checking the bites I was hanging on and what the tree sections etc were hanging on just wasn't questionable because my life, the fellas below my rigging, and the power lines, cars, houses etc below........... Nothing can beat you except for yourself in these cases.
If you go the engraving route, look for one that you can engrave yourself. I know that there are places that let people do there own. Search around for the maker workshops or even machinist shops, and drop them some cash to use it. Perhaps even pick up something cheap from ebay or craigslist etc to engrave, and pick up many pieces of metal. Just metal striplets can be as many of the same keys or duplicate keys as you want and ever need for each kind of coin that you are banking.
Consider access to water, and consider access to salt water especially. Rain and sprinklers will rust, and salt water rusts almost instantly in terms of a few years.
STONE WALLETS
YOU MAY LIKE STONE. YOU CAN ACCESS FINE STONE SLICES CHEAPLY, AND ENGRAVE ON THOSE. START SEARCHING ONLINE OR ELSEWHERE. ENGRAVE DEEP ENOUGH THAT NOTHING'S GOING TO WEAR OFF THE CHARACTERS. NO WEATHER, SPRINKLERS OR ANYTHING WILL GET THEM UNLESS THE LAND IS WASHED AWAY OR BULLDOZED AWAY BY PEOPLE BUYING UP REAL ESTATE OR BUILDING THINGS.