That's a valid point but right now the consumer space is what's driving mobile. What is Microsoft going to offer John Q Public in the mobile arena?
Isn't obvious? They will be offering SOFTWARE - the thing that they built the whole company on.
The smartphone race is over and MS didn't earn a spot on the podium, so they are done with phone hardware. They have nothing to offer that can be financially successful, and Nadella was hired by the Board to stop the hemorrhaging of cash by the mobile division that had no chance of success.
There is no room for a 3rd ecosystem - developers won't support one - and the race for the spot of "Apple's competition" was won by Android before Win7Mobile was even released.
If Microsoft wanted to have a chance to win, they needed to release a new, modern OS by the beginning of 2009 at the latest, but they were 20+ months past that deadline and still had to do 2 full resets after that. Developers weren't going to get behind that when Google's platform offered them a much better development situation and already had hundreds of millions of users.
MS missed the window and didn't have their roadmap worked out properly, abandoning users and developers multiple times. The competition was far too great to get away with that.