One of the major focuses of pre-OS X Apple was empowering the user not only with a well designed UI with a set of guidelines that third-party software consistently abided by, but also further empowering that user with programmability, with projects such as AppleScript and HyperCard. I feel that Apple has lost this particular focus, and while I personally find macOS more pleasant than Windows 10, I wish Apple had continued on its 1980s and 1990s quest of remaining “the computer for the rest of us.”
I think there’s demand for a “back-to-basics” desktop operating system that has a classic Mac OS-inspired interface and a suite of applications that complied with its guidelines.
I believe A/UX had both of these things, although it ran all the System 7 applications inside a single address space. I suspect that without this measure, many native Mac apps would not have worked.
This seems to have some details:
http://basalgangster.macgui.com/RetroMacComputing/The_Long_V...
The issue at the time is the typical Unix/X11 graphical application was not really impressive compared to what you could get with native System 7. (Which is still mostly true with modern MacOS vs Linux.) So AU/X gave you some commandline stuff... Graphical apps you wanted Mac.
Also the story is that A/UX mostly existed because of government purchasing requirements that a computer had to be POSIX-capable. Often it was never removed from the shrinkwrap.