More comparisons needed...
It would be interesting if somebody more qualified to comment than I could compare the Unix/Plan9 design philosophy with some of the better designed mainframe architectures than the System/360, such as the ICL 2900/3900 series and the later Burroughs systems.
I only know stuff about how the 1900 was organised plus a bit about the Ferranti-Packard 6000, from which the ICL1900 series was developed, IBM's AS400 and,of course the ICL 2900 series.
The 1900 was interesting because, in the middle range machines anyway, the accumulators, PC, CCR, etc were all virtualized and packaged into the first 32 words of the program. The only hardware registers contained DATUM and LIMIT of the running process, task switches were fast (just push the current DATUM and LIMIT onto the list and replace with the new task's DATUM and LIMIT. No task could address words outside its DATUM and LIMIT. Programs were easily moved within memory or to/from disk because they were by definition a single contiguous block of memory. The exact same mechanism was used to execute the GEORGE 3 OS and as an ex 1903/4 GEORGE 3 admin I can say that it all worked pretty well.
As a big plus, George 3 was the first OS I saw with a hierarchic filing system - in 1970!
FWIW I designed and wrote systems and or was sysadmin on 1900s for 6 years, 3 years on 2900s, and around 5 years on AS/400s.
I wish I knew more about the 2900/3900 machines (the 2903/4 was a 2900 disk controller in a fancy box with a screen and keyboard running a 1900 emulator and bog-standard 1900 system software), but apart from that all I know was that the 2900 ran every process in its own VM and that the underlying microcode (running on a 2MHz 6809 chip in the 2960) allowed you to run COBOL in a byte-oriented VM and (presumably) Algol 60 or Fortran in a word-oriented VM and, there was a choice of running applications in VMs containing either George 3 or VME/B.
I have a feeling that the ICL 2900/3900 and IBM AS/400 and operating systems were fairly similar, being written at more or less the same time and with one (to me anyway) a similar fault@: neither have hierarchic filing systems.