If those are their maximums, then that's the worst case for the customer right? It is these rates that would yield the biggest spread between cash and credit.
Merchants that are able to negotiate better rates inherently have less of a spread between cash and card prices right, and are therefore able to offer a better relative deal to all customers including cash customers. Walmart can better control their costs than a mom-and-pop shop, has more volume, and hence more leverage against $V. So if you're a low-income person at Walmart, you're going to be least relatively affected by card surcharges. Definitely at Costco as you point out, I think Citi actually went below cost to take the portfolio from Amex.
Even in the case of smaller shops, Square, Stripe, Adyen, etc have done a ton to level the playing field by aggregating volume and using it against networks.
It's true that Visa/MC technically doesn't keep any of the interchange -- but the interchange goes to the issuing bank, and then visa/mc turns around and charges their scheme/network fees, which basically means that the issuing bank only gets to keep about 2/3rds of the interchange (that was set and negotiated by Visa/MC). You can dice it up into the processor fee, the network fee, the whatever fees, but I believe that the general consensus is that the networks keep about a third. I don't quite remember where I read that, so if you have better info please let me know.