I'm guessing the next update in maybe a year might take some cues from the new MacBook. Like if USB-C takes off, I would be shocked to not see a new MBA with two USB-C ports on the left, one on the right and still with the headphone and SD card slots. From what I've read, the Air still has much more power than the MacBook. It's another step down from when the MBA first came out with much less powerful processors than the MacBook and MacBook Pro, but they used much less power.
I'm willing to bet each of the three MacBook lines will sell better than iMacs and Mac minis. That's why I don't necessarily think the MBA is doomed. I personally like this size a lot and would just like a Retina display. I got it for $899, which is pretty good for a notebook that doesn't weigh 10 pounds. The battery life is still amazeballs and I type so fast that I don't think I even notice the weird keyboard stuff they notice. Hell, I remember the DEPRESS ME! keyboards from the old IBM PS/2 machines we had in the school library. You needed a sledgehammer to push those keys.