There are insulated pockets of London where Longchamp’s Le Pliage bag never went out of style. Scuttle out of South Ken or Sloane Square and the Le Pliage —all rounded leather straps and nylon body — has long been an appropriate carry-all for sophisticated mothers and daughters alike. You could not, however, say that the Le Pliage was cool. Since the late Philippe Cassegrain designed the origami-inspired bag in 1993, it has experienced ebbs and flows of popularity. It would be fair to say, outside the aforementioned, well-off bubbles, recent years have seen a prolonged ebb. Now, though, the tide...