In the early 2000s, after a severe slump, McDonald’s orchestrated a major turnaround, with the introduction of its Dollar Menu.The menu, whose items all cost $1, illustrated just how important it was to market to low-income consumers — who value getting the most bang for their buck. Coming at a time of flagging growth, tumbling stock and the company’s first report of a quarterly loss, the Dollar Menu reversed the fast-food giant’s bad fortune. It paved the way for three years of sales growth a...