It's not a strategy, it's a recognition that the Windows org has decayed and they apparently don't know how to turn it around. Apparently simple projects take forever, new code they launch is often filled with bugs, different parts of the org don't talk to each other and they can't explain why anyone should write an app that targets the Windows API. I support customers shipping apps to every platform and Windows is nowadays 90% of the pain, it's worse even than Linux. Microsoft just don't care either, you can tell the devs who work on it are overwhelmed by the sheer size and tech debt levels of the codebase. Decades of compounding bad decisions have well and truly caught up with them :( This is a pity in a way, the desktop OS market could use more competition.

Nadella de-prioritizing Windows was the right thing to do for the business because it had a monopoly, so after PC sales saturated the market the best they could achieve was treading water, but also because the strategy of tying everything to Windows assumed the Windows team would continue to execute well and these things would all be mutually reinforcing. In the 90s Windows did execute well but by 2010 that had stopped, and so the tying strategy also had to stop. A better CEO than Ballmer could possibly have turned the Windows situation around and avoided the need for the disconnection, but instead it was left to drift.

It is in "everything is cloud" and "most of software runs in browsers anyway" world where operating systems don't matter .

I would not say it was by any means one or the other CEO "insightful" choice but it was more of market choosing on its own. Microsoft had to make own cloud or die that was the choice and better to put loads of investment in that. Ballmer started Azure because Amazon of course was first and Google did the same so Nadella was just playing cards he was handed by the world.

The thing is that OS is not important these days as you can apps on thin clients now and a lot of folks are spending most of the time within apps and doing nothing else.

Bad strategy for Microsoft but clearly a wining strategy for the World.