Apple Ax Product Lineup

Nowadays, we always expect Apple's Ax product lineup to be the high end with the latest chip (A7), and the rest with the previous year's products with older gen chips (A6, A5). If we look at the Mac, that's not the case. Every refresh bring a new refresh chips for all lineup, top to bottom. Eg. When Apple refresh Macbook Air with Haswell, it applies it to all the models, not just the top end. The Ivy Bridge models are gone completely (to refurbished section) instead of being delegated for a lower end lineup.

So my question is, why can't Apple do the same with the Ax products? I mean right now, there's quite a severe "fragmentation" in terms of the capabilities of the SoC, ranging from the old A5 iPad 2 to the latest A7 equipped iPad Air. I don't think this will be necessarily desirable in the long run. Coupled with the move to 64bit, I think it's in Apple's interest to have a more cohesive lineup. Let's say the A8 for the high end, and A7 for mid-end. Instead of keeping the A5 or A6, Apple could maybe use a slower A7 or something. It should still be faster than the A5, and then regardless of the device people buy, they have pretty much the same SoC. This would be great for devs too as they can push newer APIs faster instead of using the lowest common denominator (eg. the A4).

Thoughts?