POSIX is also an hourglass, right? It creates expectations on the part of apps of how the OS is interfaced to and expectations for how an OS is shaped for POSIX

POSIX is also an hourglass, right? It creates expectations on the part of apps of how the OS is interfaced to and expectations for how an OS is shaped for POSIX-compliant apps to interface to it. Details may vary, but that baseline makes it much easier to, for example, have emacs on Windows, Mac, and every flavor of Linux under the sun.