A vision this large cannot move forward without buy-in from key executives, and it is unlikely for the execution to take 9-months. Therefore, like other comments have postulated, the work had to have begun long before Nadella took over as CEO.

Let's say, for the sake of argument, that this announcement was made under Ballmer. How many of you would trust this announcement/take it at face value? Even going by this thread, there is a deep distrust of Microsoft's moves, and that is despite all the positive press that Satya has received for creating a more "open" Microsoft.

IMO, the biggest change that Satya's moves have resulted in is a change in the "perception" of Microsoft's intentions. The non-Microsoft community trusts his intentions, and by association his actions, more than they did Ballmer's. I don't blame this lack of trust, heck I subscribe to it. But attributing all of these decisions to just Satya shows a lack of appreciation for how decisions of this magnitude are made at a company like Microsoft.

And his being an executive at a corporate giant made this a very unorthodox and risky move. Having a great idea and a sound methodology is one thing; betting the future of one of the largest technology companies in the world on it is entirely different.

In large corporations, initiatives like these do not originate from the top. You're right that Nadella was probably more receptive to this than his predecessor, but a lot of the heavy lifting for this had already been done by some very dedicated teams within MS over the course of years.