Isn't that the ultimate form over function? Stop selling the previous year models just because it insults our sense of neatness of the naming lineup?
Have you ever wondered why Apple is keeping the previous model around in their iOS lineup but not in their Mac lineup? The reason is simply that the improvements year over year in the iOS models are enough to justify two price brackets. With Macs, that is much less so. You need bigger differences to over parallel lines. You also have CPU speed and RAM options within a given Mac line, something you don't have with iOS models (for good reason, even Android devices don't come with such options).