Here we have a big problem

But, there are many solutions.
The problem with any of them, the same as with bitcoin adoption, is you need enough people accepting it to work.

At some point in the future, cryptos will cross the physical/digital barrier.
Someone will figure out a way to have a solid, physically holdable and tradable satoshi.
My current idea is a piece of LCD glass, that works off of the energy that surrounds a person. Hold it a bit, and it will display a number, that is the satoshis that are stored in this tiny computer. (like a single function trezor)

The other way is to create some kind of hard to duplicate, but readily identifiable and testable piece of precious metal. Gold. (silver will be too valuable)
So, like a tenth of a gram, smashed, embossed and embedded in plastic... that is a window in a piece of paper. (looks like a dollar, but with a clear window in it, that you can see the speck of gold)

BUT!!! There is a big change happening, and cash will be dead because of it.
This is the beginning of the great migration. People will start to form into small communities that are mostly self sustaining. Trade will happen between these "families" and we won't really have a cash-on-the-blockchain, we will have some kind of barter transaction exchange blockchain ledger. Since the goods are what is actually important, the "money" is just an obfuscation layer.