FOR 16 YEARS, on her annual trips from New York to Mexico City to spend time with her husband, Guillaume Guevara’s, family, the fashion writer Olivia Villanti would visit the offices of her in-laws’ business, Gilly e Hijos — a fabric importer for some of Europe’s top textile mills — in the middle-class neighborhood of Ampliación Daniel Garza. By the time Villanti, who grew up in Rhinebeck, N.Y., and Guevara, both 39, moved down to Mexico City in 2019, the once-thriving business was in decline, a victim of fast fashion. Guevara’s 61-year-old uncle, Bruno Gilly Armand, who had taken over the company in 2011, tried selling bespoke shirting, but most of the people gathered on the street outside had come instead to tour the architect Luis Barragán’s legendary 1948 residence down the block.