As for a POSIX sub-system on Windows, well there once existed such a system that was near-compliant, but MS stopped supporting it a few years back. As for Wine, I think cygwin fulfills a similar role.
Anyway, you want your tools. You might even want your familiar filesystem and its hierarchy. I think the primary reason Microsoft deemed it worth it to implement Bash on Ubuntu on Windows was to provide webdevs access to the same tools they had on osx/linux.
All in all, I think a bit of reading on these subjects would help, because I think you're confusing the kernel with the userland.
Note: I'm faily young and inexperienced, and when I heard the "Bash on Ubuntu on Windows" announcement, I didn't really know what the meant. But learning about these bits on wikipedia/various blogs was certainly interesting.
At this point you have platforms like Electron, which run in a browser environment. Java software which runs inside JVM. Now there's even linux sub-system inside windows, etc.
I think there is a way to do it the other way around. MS put effort in creating something similar to WINE and make a special "Windows POSIX", which let's say for the next decade is released along with the classic Windows ( Same as NT before ). Where they try to emulate / rewrite the software piece by piece until the stability is reached to switch.
I don't see this as an impossible idea to me. Actually if they did something like this I will be so happy to switch back.