Earlier quoted context omitted.
They've been there and no one used it.
IIRC (old Windows NT4 MCSE here) POSIX subsystem never had access to TCPIP, which made it pointless. It was a sales point, rather than a genuinely useful subsystem. That said, I think OSs are irrelevant: people are happy to pay for apps, cloud hosting etc and MS are happy to provide it to them, Windows or not, for money.
Per http://www.unityisplural.com/2012/01/windows-posix-complianc... due to a procurement lawsuit in 1995 the Coast Guard was forced to accept a NT based "open system" bid. I don't know if they actually cared at the time, but the UNIX(TM) losing bidders certainly did, and it certainly made a big difference in subsequent adoption of NT/Windows.