For me the saddest thing is not Windows Phone going away, it's Nokia not going all the way with their Qt based Linux distro, Meego. I had a Nokia N9 smartphone, the UI was amazing. It took 5 (I think?) years for iOS and Android to sort of copy the gesture based UI, badly. Damn internal corporate politics at Nokia.
Blackberry went all the way with QNX and it didn't pan out either. There just wasn't room for a third player.
Much easier sell than random new OS.