Affluent consumers are creating a ‘bubble' at Walmart, warns retailer's former U.S. CEO Bill Simon

Higher-income consumers may be creating a frothy situation at Walmart.Even though affluent shoppers helped drive the retailer's latest beat on quarterly results, former Walmart U.S. CEO Bill Simon warns they'll be hard to keep."The Walmart experience is better than it used to be, but it's still not a premium experience. Walmart is built on convenience, cost and assortment. Not on service," he told CNBC's "Fast Money" on Thursday. "As the economic challenges abate... service will become more important than c ...