They should absolutely sell at zero profit, but it will not help their cause. The reason is because now that we have upgrade programs directly from Apple and carriers, it's more about the apps. Yes, I said it...apps. Now that people can upgrade to a $20/month iPhone 6S, nobody will buy Windows devices, especially considering that we don't have any really popular apps. People say iPhone has a lot of fart apps, but they also have every single popular app. Windows has fart apps, but nothing else. Give us phones at zero profit or even at a loss, it will not help unless Microsoft starts investing cash into devs giving us much needed popular apps.
It is no longer about the price of the device, it's all about ecosystem.