I think they'll retire the MBA either in 2016 (when the MB takes off in full), or keep it in 2016 but don't update it, and retire it in 2017.
I mean what is the alternative?
An 11.6" Retina Macbook Air, next to a 12" retina Macbook? There'd be no differentiation.
It makes no sense. And for the former to be an 'air', while it's thicker, bigger and heavier than the non-air? Makes no sense. They'll have to retire one or the other and as the MB just came back, the Air is gonna go.
And there's no way they'll be able to bring out an even thinner device, the keyboard barely has any travel as is and the space around the USB-C port (which is here to stay for another decade) can't really be any slimmer. And in doing so, they'd kill off the MB's sole reason d'etre.
Hell, even if they could made a MBA thinner and retina, the air would need to take a big step back in performance and battery life, and it's just not an option to sell a ******** laptop than the years before under the same brand.
In short, the Air line is gonna go. The new MB lineup is going to get cheaper and faster while keeping the same form factor it just launched with for years, just adding another USB-C port and a better camera, and getting incremental CPu/GPU upgrades (Skylake, more ram/storage options, and more ram/storage on the base model in 4 years, faster ssd, better camera etc).
And in 7 years the Air name return when new engineering allows to make an even lighter laptop, probably from some new alloy, with wireless charging and no ports, allowing it to be even thinner, and the MB name gets retired again.
Don't hold your breath for a new MBA. It's all about the MB and MBP now. I also wouldn't be surprised to see a 14" MB that's as small as the 13" MBA but thinner and lighter, because of thinner bezels (which are huge on the MBA). And the MBP lineup might get the same, 14 and 16 inch screens without becoming bigger due to thinner bezels and full-width keyboards.