I'm not so sure. I don't think MS sees itself as primarily a OS company. I think it sees itself as primarily an applications company, and the Windows OS (and before that, MS-DOS) was historically the best way to get everyone to use their applications. (It was a great help, of course, to have IBM basically hand them the PC software market, not once but twice--but I think if IBM had had their s*t together and had made OS/2 solid and workable sooner, MS might never have developed Windows.) But the Windows OS itself I think has always been expendable if MS could find a way to get everyone to run MS Office applications, .NET applications, etc. (and now Azure applications) without it. This might be the way that finally happens.

> backwards compatibility? Continuity with previous sysadmin tools?

If then can make Linux run on a Windows kernel, they can also make Windows run on a Linux kernel. All they need is a compatibility layer for all the Windows APIs. And MS is certainly no stranger to compatibility layers.