There is a need to distinguish between annual models in a highly competitive market. As long as the public perceives that a lot can change from year-to-year, as long as there's a "Can you top this..." feature war, you don't blur the distinction between your models, you need to shout, "latest and greatest."
If you're thinking that the way Apple has been identifying Mac models is a trend they'll extend to iPhone? It's not. The PC market is simply at a different stage, where change is more gradual and brand name and specific characteristics alone are enough to carry the sale.
PCs are now closer to the laundry detergent side of the marketing world - there may be a "new and improved formula" every so often, but that distinction only stays on the label for a short while, then it's back to plain old, "Brand X." Only specific features become distinguishing characteristics, "Fresh Scent Brand X," "Brand X Extra Concentrated...." and so on ("Air," ""Retina," "Mini").
Will iPhone reach this stage eventually? Likely, but probably not until consumer electronics has moved onto the Next Big Thing, and market share in smart phones has stabilized.
