It's kind of tragic that much of this effort may never pay off: in the process of merging platforms and restructuring API's (that started with Vista and is still going) Microsoft lost the connection to many of its developers, who went to build Web, iOS and Android apps. WPF, Silverlight, WinRT, XAML, UWP: there were too many changes, rewrites and deprecations. Windows Phone, brilliant as it is, did never reach mass adoption and all the effort that went into unifying it with Windows has probably been wasted.
I think developers prefer a crappy but stable technologies with a proven market over uncertain and fast-moving technologies. It takes many years for a new platform to mature and settle down.
Microsoft really needs to stop introducing "upgrades" that require application rewrites in Windows for the next 3-5 years, and focus on getting the Windows Store going.