Hermès is a byword for exclusivity. Its signature Birkin bag, one of which sold for $450,000 last year, cannot be bought from the luxury firm’s website or by simply walking into a store. There are neither ads in fashion magazines nor glossy campaigns on Instagram. For the not-so-famous, owning a Birkin can involve a years-long waiting list. Part of the reason for the wait is constrained supply, which Hermès manages with the precision worthy of its stitching. But another part is booming demand for all manner of luxury goodies. Last year net profits of Kering, which owns fashion labels such...