Good summary of history of Windows 10 by @DrPizza: OneCore to rule them all: How Windows Everywhere finally happened…
8 replies and sub-replies as of Jun 20 2016

Windows 10 got to a unified platform first, but they lost so much on the phone side while pursuing this otherwise-admirable technical goal.
A few months ago, I thought the merger of Android and Chrome OS would be Android with better window management, but now I'm not so sure.
They really are different beasts, even more than iOS & macOS, which share almost everything but the UI components & hardware-specific parts.
Windows 10 is the first OS where a developer can compile the same source code for x86 & ARM and have it run on desktops, phones, & tablets.
If you don't count Android, which lets you do the same thing already (in theory). Android netbooks with keyboards are a fairly recent thing.
So it goes right back to my earlier tweets: Windows 10 is going to go head-to-head with Android, and Android's going to presumably win.
Not out of any technical superiority of Android, but due to the huge disparity in software libraries and installed base vs. Windows 10 apps.
But at least Microsoft is bringing something to the table that can compete with Android N, when it becomes available later this year.