Um, never? The average developer doesn't know or understand anything about the kernel, they just like the UNIX shells and CLI environment. The popularity of the Mac (which, BTW, doesn't use the Linux kernel) is proof of that.

Windows wouldn't be windows if it didn't run 20 year old binaries with perfect fidelity.

I don't know, but I feel pretty sure that's where it's heading.

This actually has tradition, see the Irix and POSIX environment subsystems of earlier WinNTs. I think the current approach by Microsoft owes to the developments in the OS and virtualization tech space both in hardware + software over the past 20 years. Otherwise, if Linux were equally dominant in the dev/server space, we could've seen a revivial of the subsystem feature. Which would have been pretty cool, from an implementation POV, in its own way.

Won't happen for the same reasons for Google to build Android replacement.

Isn't that exactly what android is—an alternative UI on top of Linux?

correct but the mess Android have is somehow direct result of using Linux's kernel, read about Fuchsia and why it was created.

It is more than that, because Linux isn't exposed as such to userspace.

One Eric S. Raymond has the same hunch as you. See his latest post http://esr.ibiblio.org/?p=8764.

The new edge is just chromium, which already ran on Linux. It seems like a stretch to suggest it is emulating the windows version rather than just being a native Linux app