This excellent article from QZ says it better than I could. It didn’t have to be this way; I blame Ballmer.
Microsoft still needs to have a mobile platform, but only as an implementation of Windows 10, for use in small devices, with or without telephony. That telephony-optional approach might actually help, for example, eventually paving the way for a “Surface Mini” or equivalent device that does telephony.
The Week takes it a step further, prophesying that by failing in mobile, Windows itself is ultimately doomed.
(I’ll be posting a longing entry on my final thoughts, soon.)