The cryptocurrencies only remain as secure as your PC (or hardware wallet) is. And those things are only secure because they have the right software/firmware in

The cryptocurrencies only remain as secure as your PC (or hardware wallet) is. And those things are only secure because they have the right software/firmware installed on them. And the reason you know you have the right firmware is because you trust the manufactures to put the right one in, and trust store/postal service not to tamper with it. Once you are in the domain of "Anonymous $1 cards which people swap with each other", you no longer have this option. The obvious thing is to have corresponding entry in online database, but then anyone who wants to verify the card value will have to access this database -- so your privacy disappears. I am not saying that it's impossible -- people are smart, something can likely be figured out -- but it is not obvious to me those cards can be made harder to forge than cash.