Sure, Linux doesn't have a total monopoly in every domain. Sony used FreeBSD as the basis of the OSs of the PlayStation 3 and the PlayStation 4, for instance, and the work they did will never be contributed back.
I don't see any proprietary Unix seriously competing with Linux any time soon though.
The OSes that I have listed have all BSD style licenses, nothing proprietary about them as such.
As for competition with proprietary UNIX, probably not against Aix or HP-UX as they survive on maintenance contracts for big customers like banks and telecommunication companies, but Linux is still no match for high integrity computing OSes, many of which are micro-kernels with POSIX userspace available as possible API.
> The OSes that I have listed have all BSD style licenses, nothing proprietary about them as such.
Yes, I know - but the software which ends up getting deployed/used/sold will be proprietary forks, right? That's the whole point: the licence is friendly to proprietary forks.
> Linux is still no match for high integrity computing OSes
> And thus, you should be skeptical of any claims of Linux providing security.
When did I say Linux has perfect security?
I presume the point you're trying to make is that different operating systems, with different architectures, can beat Linux in various regards. Of course this is correct. But for a general-purpose multi-platform Unix-like OS, Linux is king, and will be for the foreseeable future.
I don't see any proprietary Unix seriously competing with Linux any time soon though.