Apple is reportedly trying to unify iOS and macOS app development. The same problems that challenged Microsoft also apply to Apple. But Microsoft is way ahead in that game, even if it's far behind in mobile.
Microsoft bet big on Continuum's ability to give a phone a desktop experience. Modernized Win32 apps via Project Centennial and UWP are critical to that scenario, so getting developers on board with Windows S is a necessity.
Windows phone developers have endured the same tumultuous journey fans have. Consequently, many have given up on Windows. What must Microsoft do to win them? For one, Microsoft can no longer ignore why some developers don't trust them anymore.
The future of Windows 10 and the Universal Windows Platform (UWP) depends on more than just PCs. With Microsoft missing its goal of getting Windows onto "one billion devices" by half, where does Windows 10 go without phones?