> I am genuinely curious who can actually threaten Visa (I do not think it is Valve).

Visa is a clearing house whose members are banks. Think of it like a payment router between issuers (banks) and processors (banks).

Only sponsored organizations can directly use the "Visa rails", where "sponsor" is defined as a bank, a bank subsidiary, or an entity previously sponsored by one of the other two.

This is also the case for MasterCard and Discover. "Traditional" American Express is different though.

> Amazon, Walmart, Target and then increasingly unsure.

Those merchants use banks or one of their subsidiaries for processing credit card transactions. Most large merchants do as well in order to minimize their discount rate as well as other transaction fees. Smaller merchants often use ISO's or VAR's for business specific reason, knowing both ultimately transact with a bank or one of a bank's subsidiaries.