My impression is that it's taken a while to achieve primarily because of leadership roadblocks, which Nadella changed.
I've heard this from MSFT employees and I believe Ballmer made a post-retirement comment in an interview about the same: that this all started after he left.
Oh I totally agree that upper management roadblocks slowed things down and that Nadella is a breath of fresh air compared to Ballmer (though I do have a soft spot for the man). But the seeds were sown by division staff such as Guthrie (and Hanselmann, Phil Haack etc) as far back as 2007, long before Nadella became a rising star, this isn't all as a result of Nadella's Perestroika and Glasnost. That said there may have been informal conversations between Guthrie and Nadella over the years speculating as to how they could change direction.
