Except in certain cases in the short term, coins are terrible for investing (numismatic value buying and selling), but remain excellent as money since the dawn of recorded history - and probably further back than that. It's why metal coinage worked for 5000+ years, despite people shaving and filing bits off them, or otherwise counterfeiting them, and were only defeated by modern monetary policy and unconstitutional fiat decrees backed up by violence.

"Imagine a world where you had to have an expert authenticate each coin during each transaction."
I already live in that world, but people don't use coins, they use rectangular plastic debt notes, and those must be authenticated during each transaction. Cashiers nowadays must be specifically trained to spot counterfeits, and most bills are also passed under a special lighting device to help authentication. I was a cashier back in my youth when things were a bit less digital, but we had the same issues with counterfeit notes. We've always had to deal with money scammers. It's not like digital "money" can't be shaved, filed, and counterfeited as well.

You talk like there would be a lot of coins to authenticate, but we now have coin-reading machines, which weigh each coin before accepting them. I can (and have) dump a large bag of nickels and dimes, and within 20 seconds it has sorted, weighed, scanned, and authenticated every single one, deducting the total from my grocery bill. Don't you think such machines would be put into use to authenticate each silver or gold coin, if they're already doing so for copper, nickel, and steel coins? Come on. Authentication is 100% accurate and instant with the right 1990s technology. We can already do it. Imagine if there was economic and political urgency behind the problem? I'm certain we'd be able to figure it out. But there's no such urgency, in fact, the push is the other way, to melt coins and replace them with virtual dollars and cents. We have purposely removed all value from our money, and made it worthless, under the guide that valueless currency will solve our financial problems. In reality, it's the cause of our financial problems.

And is paying with debit or credit cards faster? That's your implication, anyway. But I don't think even THAT is true. I'm in and out from a cashier when I've got coins. $3.55 you say? That's five coins. Boom, they're on the counter, no I don't need a receipt, thanks, bye. Meanwhile buddy is paying with his debit card and everybody is backed up behind him as he cups his hand over the security screen to punch in his PIN. "It's so fast and convenient" was the lie they told us to accept debit and credit cards for everyday purchases like coffee. We ate it up and either didn't notice, or didn't care, that it wasn't any faster or more convenient.