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The thing is, we use now a form of Crypto. Each time you use your Bank Card you never see the cash, just numbers on a computer screen.
Quantum computers will render block chains useless. Watch this space for the emergence of quantum computers.
But "cash" is intrinsically worthless too. Just base metals or paper with controlled stamping on them. It's only the bank stating it's value and the subsequent acceptance of the population that it can be used to buy goods that gives it value.
UK bank notes are basically just an IOU, they state "I promise to pay the bearer on demand the sum of X pounds"
If you've not read it, Making Money by Terry Pratchett is an excellent read and really delves into the arbitrary nature of cash and the concept of held wealth.