That would have been difficult since Plan 9 predates each of those other systems. Also Plan 9 took place in Bell Labs as a research project, was based on their own UI research, and was not intended to be a commercially familiar UI. There are interesting ideas in the write ups about the UI that could be applied in nearly any UI today.

Not an expert in OS development whatsoever but I do know that it's not intended for common usage, so how its UI looks is not something devs would take much care of.

Though I do know that it puts some strong emphasis on mouse usage, something that for someone that grew to use the keyboard a lot like me (ironically, as a graphic designer) seems to be really awkward, to say the less. Its strengths seem to be its overlaying concepts and that it intended to be "the next gen Unix" - alas it won't take over for a myriad of reasons, and some would argue Unix-es have already borrowed some of its concepts for themselves.

Forgive my naïvety, but couldn’t X and CDE be ported over?

I thought there was an X server for Plan 9 (edit: yes; https://plan9.io/wiki/plan9/x11_installation/index.html ), but it kinda defeats the point unless you use something like https://github.com/gerstner-hub/xwmfs on it. CDE... something like CDE probably, but actual CDE would be painful and really defeat the point unless you ported its IPC to a Plan 9 native version.

You could, but if you want Unix, you already know where to find it.

Not trivially. There's a lot in how Plan 9 is put together that makes this a monumental effort.

(That said, there may have been an X server brought in at some point, but don't quote me on that. That's the least of anyone's problems in undertaking a CDE port though.)

Plan 9 was heavily influenced by the Xerox PARC Mesa/Cedar interface, which influenced Wirth’s Project Oberon. I forget whether Project Oberon directly influenced Plan 9, but I’ve heard people argue that Rob Pike, one of the leaders of the Plan 9 project at Bell Labs, was heavily influenced by Wirth when it came to programming language design, even if the syntax was closer to C instead of Wirth languages like Pascal, Modula-2, and Oberon. With that said, there are major similarities between the interfaces of Cedar/Mesa, Project Oberon, and Plan 9.

A few years ago I thought about what it would take to implement a more conventional desktop GUI on top of Plan 9, but I’ve oscillated back and forth between wanting Plan 9 with a Mac-like desktop versus wanting a modern Lisp/Smalltalk machine (with object-oriented underpinnings instead of Plan 9’s “everything is a file” interfaces) with a Mac-like desktop.

You see Oberon's influence on how ACME editor works, and later the OS dynamism enjoyed by Inferno and Limbo, that everyone usually forgets about.

Inferno is not a successor. For example you can have Golang for Plan 9 but doesn’t make much sense on Inferno. You would even run Inferno on Plan 9 on some scenarios. I suspect most people who know about Plan 9 also know about Inferno, but it’s just a different thing, does not supersede it in general.

Plan 9 => Inferno

Alef => Limbo => Go.

Being able to target Plan 9 with Go doesn't make sense, that isn't how historical evolution works.

I suspect you would have to port Go to run on Dis, the VM. C is for the OS. It’s a different design, without mmu. Plan 9 is still a classic OS with hardware isolation

The UI is called Rio. It’s simple yet functional and the plumbing thing is kinda cool. The thing that irks me with Plan9 (I have tried 9front) is the lack of tab completion on the shell. It’s easy to input garbage on the screen and contrary to most modern shells where you can just skip a line you need to use your mouse to put the cursor back where the prompt is.

Of course lots of it might be skill issue.

Ctrl+f for completion, ctrl+b to warp the cursor back to the prompt.