[truenindb]

POSIX programs were portable enough that Microsoft had to become POSIX compliant before being granted the operating system monopoly rent by the free market.

[pjmlp]

"Portable" until you rely on signals behaviour, file systems, threading, file locking and other stuff left for the implementations.

Windows NT POSIX subsystem was for ticking check box on DoD contracts.

Had Microsoft been half serious about POSIX in NT like they are nowadays with WSL, Linux would never had taken off.