Genuine watch sales will follow the economy much like fine art, or exotic car sales do. As long as there is desire for a specific genuine brand, I don't think rep sales will interfere enough to matter much at all. Do prints, or kit cars impact the art or exotic car markets? Anyone buying a monet or Murcielago is going to have definitive proof they are the genuine article.
Lest we take into account the precious stones and metals the gen's are made out of which will always hold their value, for much of the same reason the watch "brand" itself will- desire, with of course a splash of human greed.