"Why invest in the huge manufacturing, inventory, shipping, and advertising logistics for what is rapidly becoming a niche (I need one just for the gym) kind of product? "
According to your theory then, Apple should abandon the Mac Pro.
You misunderstand product life cycles. The Mac Pro is a product that is not in decline, unlike the iPod, which has been for a long time. The Mac Pro is a niche product, but one with a very large profit margin in an industry that will not transition into decline for a very long time.