Being progressive rather than universal
Microsoft was not short of ambition for its tiled interface and One Windows strategy. Microsoft was doing what no other tech firm had done - uniting mobile to desktop/laptop PCs in one experience. Apple had two OSs available - IOS for phone/tablet and OSX for its desktop/laptop users. They were different and had different interfaces. Google had two OSs - Android for phone/tablet and ChromeOS for its Chromebook devices.
Of course, even One Windows doesn't mean exactly the same OS but it does mean components have such similarity that the interface appears the same to users and, with little modification, apps can run on all platforms.
Unfortunately, the app platform UWP (Universal Windows Platform) was hobbled from day one. Microsoft had a poor quality application store, ever-changing developer tools and standards and a Store that only worked (on the PC side) for Windows 8 and latterly Windows 10. While people used legacy versions of Windows such as Windows 7 and XP the apps were "invisible". They also kept rebooting WindowsPhone from 7 to 8 then 8.1 and 10. Each iteration became a year zero where developers were faced with re-writes.
UWP also became fragmented. Developer "bridges" allowed conventional WIN32 legacy apps to be delivered in a UWP "wrapper" to aid installation. There was also a wrapper so that websites could be delivered as an "app" but were just launching a web connection. UWP became more of a way of distribution via the Store and adding notifications rather than being universal. Microsoft's interest in phone waned as they retreated from successful markets turning them into unsuccessful markets. It literally appeared their ambition was to achieve zero sales in mobile.
You can't have a "universal" platform if your universe consists of just the PC!
Just as Microsoft abandoned mobile devices, affected greatly by the dearth of good apps, PWA arrives.
Progressive Web Apps (PWA) are the force that makes apps unimportant. A PWA is a website that has components that run on a smartphone like an app. For most people they will act like an app. They can be pinned to home screens and may even work offline. Mechanically a PWA has "manifest" of components that make it work online or potentially offline using the web.
There will be some applications like CAD, Photoshop and games that will need to run natively on the device they are coded for. However many phone "apps" can become progressive and can work across any device. Google and Microsoft are onboard with the concept. The next Windows 10 update with Edge will support PWA. The Windows Store will be able to wrap PWA "apps". Maybe even Windows 10 Mobile devices could run PWAs and then have parity of function with Android devices.
There is irony here. Could it be that Google backing for PWA makes Windowsphone viable?