On what Microsoft should or shouldn't do: Microsoft took a big gamble with the NT kernel, ultimately killing the Win9x line, and that was clearly a good thing by the time they got to WinXP, with many people still running it some nine years after its release. With modern smartphone hardware, there's no reason Microsoft couldn't move NT onto a smartphone in precisely the way that Apple moved OS X. (I'll argue that there's nothing wrong with the NT kernel, but rather with the behemoth environment that's grown up around it. Android has done what looks to be a nice job of redoing the Linux userland without monkeying with the kernel, so far as I can tell.)

Of course, as you point out, Microsoft seems to have this thing about producing products that have their own software stack from top to bottom. For example, there was their famous effort to move Hotmail to a Windows platform, and Kin can be seen as moving Danger to a Windows platform. Microsoft could well have left both of those alone and focused on evolving things on their original platform, but they didn't.

Mostly I just see Kin as a spectacular management failure.