A ruthlessly hyperefficient competitor, like Walmart, can put everyone else out of business. The result is a monoculture, which likely to be less stable, robust, and healthy than a diverse ecosystem.
John Quarterman points out the vulnerabilities of monocultures.
According to http://www.walmartfacts.com, Walmart is "the nation’s largest private employer" and "the world's largest retailer."
Virginia Postrel comments on the unexpected impact of Walmart on the US economy.