It's quite the opposite, because they're there not to sell couture, they're to sell "illusion" brand images to the commoners so they can sell more makeup and perfumes. Because a normal woman may not be afford the Chanel suit Penelope wore but she can always but the lipstick or the perfume that she the face of. Having celebrities wearing couture sometimes can be back fire because, Karl said few years ago, a couture client saw the suit that she ordered have been worn by Nicole Kidman and she cancelled the ordered immediately. But they make enough money from makeup and perfumes so couture is more like a brand building practice even if they still make profit with couture.
With Hermes they don't need "human celebrities". Their Birkin and Kelly is their best brand ambassadors. Because unless you're really important people or close with someone in the company, the chance you can buy those 2 bags is really hard. In order to buy those bags you need to invest a lot of money in Hermes like buying their RTW and accessories (silk scarves, ties, belt, etc...) and that how they make money. Hermes use the images of Birkin and Kelly to sell other products. As long as Birkin and Kelly keep being prestige Hermes can always make money.