I'm not sure that's true. Windows did just fine siphoning off marketshare from the commercial Unixes without a POSIX subsystem, and Linux's original supporters

I'm not sure that's true. Windows did just fine siphoning off marketshare from the commercial Unixes without a POSIX subsystem, and Linux's original supporters were people who wanted a free Unix they could tinker with, and it started gaining market penetration when those people started getting sysdmin jobs and Linux was what they were most familiar with. I can't imagine they'd have used Windows instead just because it had a POSIX userspace- remember, this was when Microsoft was peak Evil Empire.