Perhaps there is more than one retailer you can mention here accepting Hive Dollars? You can make donations in Hive dollars certainly. Unless you're running a pretty demanding internet application server through Privex, there really is no reason for someone to be spending hundreds of Hive dollars a month.
I am of the opinion that 28 million is the amount of dollars we need or an upper bound because if we needed more dollars, the value of the dollar would go up slightly and the mechanism would compensate to create more dollars.
Should witnesses take away interest payments, the number of dollars will go down in a similar way.
Long before the Hive fork, I had something that accepted Steem Dollars and people mostly tried to sell jpegs. I guess it was too early. Yet @privex has a payment system that takes multiple currencies including HBD and even integrates Hive Keychain so the user doesn't have to fuss with memos. If someone wants to accept Hive dollars, they could contact me, or better yet contact him. The system I had set up was quite primitive.