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.
This visualization challenges the claim that Yahoo! mail has a very strong user base in Asia. The volume of email delivered in Asia, Australia, South America and Africa looks negligible compared to the volume delivered in the USA. Email volume is an indirect indicator of users, but it is a good indicator of "usage" - spam or otherwise. It's not a stretch that there wouldn't be usage without users, ergo "indirect".
If I were a number of the Yahoo! mail BizDev or Accounts teams, I would ramp up my marketing efforts in the areas that are sans bubbles on this visualization. This is a great tool for marketing and marketers (of the service) alike. Kudos to Yahoo! for making it without Flash.
For loyal Yahoo! Mail users like myself, this announcement reaffirms Yahoo!'s focus on, and commitment to, their most popular services. I also like the unified experience on all devices.
The CEO's post shows her involvement in these changes. The changes demonstrate her desire to rework the underpinnings of services rather than revamping the user experience for marginal gains. The former infuse the right elements into Yahoo!'s culture; the latter serve as window dressing and don't help to mitigate/fix the underlying issues that have handicapped Yahoo! over the last 5 or so years.
I am looking forward to future updates to Yahoo!'s services.
I welcome the day when this practice, or something similar, catches on in India. Almost every Indian politician has a criminal record, is a thief who siphons money into tax shelters, and does all of this with impunity. Sadly, this trickles down to all the government employees, cops included. This kind of citizen action will expose the crooks for their wrongs and create accountability. Shame is a powerful motivator for change...