The current Air is already thinner than the 12" (0.44" vs 0.52") it just doesn't taper. One consideration is that miniaturisation is expensive, as are custom features like a terraced battery which underpinned the old MacBook offering a decent Mac experience, so I probably wouldn't expect the cheaper device to be the one to go there. Perhaps Apple will be able to shave a bit more size and weight off of future MBA models, but again, a bit of a ceiling with the relatively low price point they need to hit.