Microsoft's strategy is to be cross-platform, which is why it has dozens of apps on iOS and Android, and supports Linux on Azure.

Microsoft's Windows strategy is to be cross-platform, with Windows 10 for "internet of things", phones, tablets, games consoles, all types of PC and servers.

Windows phones didn't sell well enough and the hardware lagged what was needed for Windows 10 (eg Hello and Continuum). However, there's still an ARM/smartphone version of Windows 10, so that door's not closed yet. There are always rumors about a business-oriented Surface phone

Also, Windows 10 was and is free for smartphones, so there's still room for Asian manufacturers to have a go. Even if they don't ship many units, it provides a fall-back if Google gets too aggressive on Android.