well, those are the first two 'E's of the picture, if this is the case. with the latest edition of Windows 10 (S, i think?), microsoft have already prepared the stage for the third.
WSL doesn't look to be "extending" Linux; in fact, it's all about providing the exact ABI Linux itself provides, and running Linux applications and operating systems without requiring any porting. There are no e.g. WSL-only syscalls exposed to Linux applications, such that Linux applications would benefit at all of being aware of WSL or targeting it. As far as the code running under WSL is concerned, it's just POSIX.
To me, that's actually a strategy strangely opposed to classical Microsoft EEE: this move commoditizes the Windows platform. There are many developers who will now see Windows as "just another POSIX", and will experience less of the unique, Windows-only parts of the platform, making them less locked into Windows as a whole.
There's no plausible end-goal here where Windows would ever be given mindshare as "the Linux," in order to start on the "extend" phase. In fact, more likely, Windows IT people will be enabled and encouraged to switch to "standard" services like nginx instead of IIS, or Postgres instead of MSSQL—at which point they'll realize they've eliminated all Windows lock-in and they're now free to leave the ecosystem!
Yet.