I think you vastly overestimate the need for Walmart in a person's life if you think they will actually switch their credit card to Mastercard with their bank just so they can shop there.
Cashiers will almost assuredly offer a Walmart branded MasterCard or Amex if they were trying to buy with visa.
Bingo- Walmart has both eliminated visas Costs and developed a new revenue stream. And I imagine their quants know how many of their visa customers have alternate credit cards or debit capabilities.