Since this once-sleepy stretch of Mexico’s Caribbean coast began its transformation into a glitzy resort community 50 years ago, it has thrived by attracting millions of vacationers during its summer and winter seasons. These days, however, there are long stretches of the deserted beach. Hotel occupancy has fallen below 30%. Shopping malls are empty. And the tourism industry that helped pull Cancún and its people out of poverty and put them on a more prosperous path is struggling. …