> In any case, as the investors buy the product they eventually have to sell it, meaning over the long run demand and supply are good measures of "actual" usage and production.

That's not necessarily true. It depends on the product and industry, but it's absolutely possible to horde products for long periods of time (see diamonds and gold) or to choose to destroy products rather than sell them at a price you don't like (see public transportation in many US cities in the 50s, or fancy food items).