If you're going to upgrade with the next version, you shouldn't eat too much of a loss, especially if you can time it right and sell your 2015 before they introduce the specs on the 2016 model. Then again, you may be without a computer for a while, especially if Apple takes a month or more to roll out v2.*
Demand for it may even go up for it as they actually make their way into the wild. I've got the only one I've seen in my medium sized city, and I look for them every time I get an overpriced coffee. Right now, I think they only people who are even aware of them are apple enthusiasts.
In the meanwhile, let us hope and pray does become the new standard for usb so devices and adapters will become cheap and plentiful. That would probably overcome the biggest objection/concern to people coming late to the party. If people can buy the equivalent of the apple super dongle for $30 instead of $79, it will almost become a non issue.
In 2016, nMB v2 will have the skylake cpu and gpu with some performance gains, a better camera**, and maybe one other software feature that Apple will somehow tie into the new hardware. This, or right before it, would be the time to sell your nMB to minimize loss. Once the specs improve and Apple drops the nMB price to take the place of the discontinued MBA line, good luck finding a buyer.
*Honestly, I don't know why I'm saying "if" at this point.
**Because we all know damn good and well that Apple will include a better camera in the next model and hail it as a triumph and a breakthrough. Sandbagging.
