For a decade, Walmart has reigned as the nation’s biggest company by revenue. Its sales last year added up to $648 billion—more than $1.2 million a minute. That status comes with benefits. It gives Walmart power in negotiations with product manufacturers and in dealing with government officials over policy issues. It’s also a point of pride: Job postings often tout working at the “Fortune 1” company as a perk. Its reign is looking shaky lately. If current sales trends...