Comment by danielmarkbruce

Not a complete response, but:

Checks are riddled with fraud and traditional bank-to-bank protocols(like ACH here) are notoriously slow and difficult to navigate for disputes. Visa and Mastercard aren't just selling a network effect, it's instantaneous clearing guarantees, active fraud prevention, and dispute resolution. The "cut" merchants pay funds the risk management layer that standard routing protocols don't provide. Even new systems like Brazil's PIX or the US's FedNow require a centralized state authority to mandate the clearing rules and manage the trust boundary.

Walmart can afford to vet lots of suppliers. A mom-and-pop shop cannot, so they buy from a massive centralized distributor who aggregates that trust for them. Supply chains have distributors all over the place, and they are concentrated in almost all cases.