"Windows Mobile US smartphone market share, based on install base not unit shipments, is considerably higher than its successor -- 4.1 percent versus 1.7 percent for newer Windows Phone.
Commenter aretzios blames Windows Phone for Microsoft's misfortunes: "Microsoft signed its death warrant in the mobile space since the day it decided to abandon WinMo 6.5 to move to a totally incompatible system...From 25 percent of the market, Microsoft's share declined to 1.5 percent as WinMo users moved en mass to Android. Sometimes, when one breaks backward compatibility, there are consequences".
During the Bill Gates era, and even during Steve Ballmer's early tenure as CEO, Microsoft made backward compatibility a top-design priority. But the company started backing away from that tenet with Office 2007. Where there are dramatic UI changes, like Windows Phone, Windows 8 and Windows RT, backward compatibility isn't assured at all."