To revive mobility, Microsoft would have to spend billions$$$. Not going to happen with a company that appears to be interested more in enterprise business than consumer.
Secondly, even if they did do a reboot (again), you still have the issue of apps. Can't imagine any company spending a dime on what would be the 5th reboot. Microsoft couldnt get longtime partners to create apps for printers, which would be my starting point for apps.
I just can't see a path forward for another attempt at mobile. A Surface phone could have been a flagship phone, with windows phones as lower priced units. Do i think there is a space for a third mobile system? Yes. It just doesnt unclude Microsoft.