Gordon Cox Theater Editor Ask Cameron Mackintosh if “The Phantom of the Opera” will be back, and he doesn’t play coy. “Of course it will return,” he says. “All the great musicals do.” But the British mega-producer, whose credits include “Cats” and “Les Misérables,” also speaks candidly about the forces pushing “Phantom,” the longest-running show on Broadway, to end its record run on April 16 after 35 years. Even before lockdown, the Andrew Lloyd Webber musical, which had fallen into a boom-or-bust pattern of sales fluctuations based on seasonal tourism influxes, seemed to be reaching the end of its lifespan. “Over the years, the number of good weeks at the box office started to shrink,” Mackintosh says. “Some of the really bad weeks, we lost a lot of money, particularly in New York.”