That's only some of the consumers. Even if there's 5% less business/revenue to Walmart as a result of this decision, that's a huge hit.
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.