I say this with great care as I do not want to launch a flamewar.

If you do not consider Linux with namespaces an OS (because of fragmented userland): Would you then consider FreeBSD with jails or Solaris with zones for fully fledged?

If you still consider those flawed (maybe because thet do not force you into jails/zones) should we at least no consider OS/390 or z/OS as proper operating systems to that/your (not meant inflamatory!) standard?

Yes. Though you do not mention them directly DOS and Windows has ruled the world for years and they opened the door for the nasties. But they were not all there was - only the popular/easy choice. Everything is a trade off.