These systems do things that Linux just can't.
Linux is no RTOS, nor does it have a pure microkernel architecture. It can't do anything where latency guarantees are needed (hard realtime) nor high assurance, nor any actual semblance of security.
I suggest these posts: https://microkerneldude.wordpress.com/2018/08/23/microkernel... https://blog.darknedgy.net/technology/2016/01/01/0/
There's far enough technical reasons to ditch Linux for a cleaner design.
I don't see any proprietary Unix seriously competing with Linux any time soon though.
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.
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
Sure, VxWorks is safe.
That goes beyond strawman.
Try reading: https://microkerneldude.wordpress.com/2018/08/23/microkernel...
For as long as Linux is a monolithic design, the whole kernel is going to be part of the TCB.
And thus, you should be skeptical of any claims of Linux providing security.
It's a strawman when I agree with someone?
> 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.