It was, to be sure, an unlikely megahit. When Downton Abbey first graced U.S. public television screens more than a decade ago, the Masterpiece series seemed tailored for the kind of Anglophilic niche audience that thrills to adaptations of Jane Austen and E. M. Forster (not that there’s anything wrong with that). Executive producer Gareth Neame and creator-writer Julian Fellowes offered up a post-Edwardian England populated by nobles—the Earl and Countess of Grantham, their daughters and his formidable mother; along with their butler, housekeeper, maids, footmen, cooks, et al. It all seemed genteelly out of step while Americans binged on sociopathic antiheroes, rampaging zombies and reality stars. But Downton Abbey became, in 21st-century parlance, a thing. The interwoven narratives, enacted by a spot-on cast, weren’t period pieces but timelessly relatable…