Well, irrespective of how some Lumia 900 customers feel about this move, I'm pretty sure that it's in Microsoft's best interest to do this. Having the same kernel and APIs available across all their devices provides for a very compelling developer story: write to this platform and you can easily deploy your app/game/service to phones, tablets, PCs, and console. I have little doubt in my mind that the successor to the Xbox 360 will be built on this same platform, especially since Dave Cutler is apparently working on it (the next Xbox that is).
For a while now Microsoft has suffered somewhat because they placed too big of a value on backwards compatibility. Sure, backwards compatibility is important, but it's not as important as being able to move forward in meaningful ways.
I look at this and see Microsoft "pulling a Apple".