[chrisseaton]

I wouldn’t get why POSIX is the only way to design an OS.

[arendtio]

It is not.

But that way you could pretend to have an objective definition of the term 'real OS' and exclude Windows while including most other major OS.

[rbanffy]

It’s not. It’s an API. IBM’s zOS has a POSIX-compliant Unix subsystem and it has absolutely nothing to do with Unix under its hood.