In response to falling sales of diet soda, last year PepsiCo changed the sweetener in its main calorie-free beverage, Diet Pepsi. Noting the health concerns that some customers have about the original sweetener in Diet Pepsi, aspartame, the company switched to a different sweetener last year to try to reverse a sales decline. Now sales are declining even faster. Customers had time to adjust to the idea of Diet Pepsi without aspartame, since rumors about sweetener tests have appeared in these very pages since 2012. The version of the beverage sweetened with a blend of sucralose (Splenda) and acesulfame potassium...