The US probably could get by with just $1 and $5 coins.
The Japanese still love their cash and coins. And even though they have a variety of NFC payment options and credit cards are not as terrible as they once were, their adoption rate is slow.
Do you know any? If I could vet its security and """reasonably total""" privacy, the idea could be tempting - e.g. to buy a cheap device to only do monetary transactions. I am not sure, first of all, how you would transform cash into credit into a personal device.
* Country-specific: prepaid cards like Suica in Japan, purchasable with cash from ATMs.
* Not that cheap: offshore bank account.
* Not legal: crypto card with "borrowed" KYC.
So basically unless you're lucky to be living in a country with a privacy-respecting culture, you have to act like a criminal to regain the basic right to privacy.
Because I also know of systems which are related to banks, but explicitly come from your account, invisibly to the seller - but the bank knows everything.
I have seen anonymous pre-paid cards which had in the back the space for a signature, and were accompanied by instructions such as "must be signed to be valid".
Metal coins seem like a case of holding onto tradition despite losing the original reason for the tradition.
Plus, quarters are better to flip to make decisions than bills.
They're also much easier to handle when you need many different ones, like when you're trying to assemble an exact amount. I can just look into my wallet and pick out the coins I need, whereas with notes, I'd have to spread them out like an awkward floppy hand of cards and especially if they weren't distinct enough scanning through and finding the right ones sounds plain annoying.
(Plus, 2 dollars is like a quarter 50 years ago....)