While it was expected that Microsoft wont provide Windows 10 on ARM to older Windows Phones like Lumia 950/XL and HP Elite X3, but some loyal fans were hoping to get it. Once again Microsoft disappoint their loyal fans and Joe Belfiore confirmed that old windows phones wont be getting Windows 10 on ARM. He provided a technical explanation why existing phones wont be getting the Windows 10 on ARM during the Windows Insider Mixer Broadcast (47:20 minute mark) :
The Windows 10 on ARM effort is about enabling the PC experience on devices that are built on ARM so that they’re connected all the time and have great battery life. So the experience is a desktop PC experience, it’s not a phone-like experience. For phone-like experiences on ARM, we have Windows 10 Mobile. What Windows 10 on ARM is, is a desktop-like experience so that you get the battery life that ARM processors tend to have.
The other thing that’s interesting about this is that in all cases where you build ARM software, it’s not so easy to just put it on another device. The OS intermingles with what’s called the BSP (board support package), and that’s how the software talks to the SoC. There’s special work that has to be done to get the OS talking to the BSP, talking to the SoC, so it’s not like the PC ecosystem where we write an OS in the same binaries that run on lots of different PCs. With ARM devices with SoC, you have to more closely target it. So that is often the case why it’s more challenging for us to get updates to a wide range of ARM devices. They’re all quite different, which adds delay, and as the number of people using those devices gets smaller, it unfortunately makes less sense for us to invest more time and energy in that.
Its quite understandable why Microsoft decided to scrap older windows phone but fans are not going to take it lightly since Microsoft has dropped balls on its fan many times before..