The argument goes both ways -- I've spent hundred hours of my life learning POSIX, and if my boss wants me to run a POSIX program in Windows, I'm pretty much doomed. (I know a bit of Win32 API if that helps...)
"Open" does not mean "supported on all major platforms". It only means "can be supported by other vendors if they choose to". And if you choose a language or technology that is out of fashion, it doesn't really matter whether it is open or not.
And yes, I know Microsoft Windows is POSIX compliant.