So with phone API's in Windows 10 suggesting MS may make full Windows 10 into a 'Surface phone' they will still have the problem that there aren't that many 'Metro' apps to use on the phone from the Microsoft store. And although ARM versions of Win10 can run Win32 software under emulation, so that give millions of programs that are available. At lot of the software written for the Window desktop is going to be fiddly to use on a phone screen and not make for a good user experience.
Plus after being shafted several times by Microsoft with their mobile strategy they will find i difficult to get developers and business back on board, since MS might pull the plug again in another couple of years if sales don't meet their expectations.
I have just 'downgraded' back to Windows 7 on my laptop when i fitted a replacement hard drive as I found Windows 10 was more resource hungry on the same hardware than 7 or Linux. Often the CPU and hard drive were busy looking for Windows AV and security updates for 5 minutes after the computer has supposedly booted to the Windows 10 desktop, even if i had just rebooted after it has been on for hours. So not the sort of experience i want from a phone.