I have a play with Inferno in a VM some years ago. (I can't even find an ISO of it now, which is something that needs fixing badly, alongside a ready-to-run version for Linux on x86-64.)
Oberon, and thus Plan 9, spring directly from the era of the original Xerox PARC Alto. Their design comes from before Apple standardised the GUI in the early 1980s, with standardised menu bars, standard window controls in the title bar, the idea of dialog boxes, plus ones that have standard buttons in standard places.
(You know -- all the sort of stuff GNOME is busily removing again now.)
Microsoft mimicked this design in Windows 1 and 2, DR aped it in GEM, IBM in OS/2 1.x, etc. All later GUIs are informed by LisaOS and MacOS 1.
Oberon was before that and it has none of that stuff. Plan 9 loosely mimics Oberon and so it doesn't, either.
Result: it's very weird if all you know are post-1990s GUIs.
Inferno modernised the Plan 9 UI and it's quite usable by comparison. But Inferno is even more obscure than Plan 9.
FWIW, The A2 OS modernised Oberon and it's much more usable, too. Both are still weird but they're vaguely recognisable and you can just start clicking on stuff.
I'm sure this could be implemented using keystrokes as well and the source code is of course available. However, I doubt such a change would be accepted upstream. You could say it's a central point in the "Plan 9 philosophy".
You could also read the process memory directly by a supervisory program, like a debugger, to extract or insert text directly. It would be like idea implantation.
Does that make sense?
I don't think the mouse chording is that essential to Plan 9. It's how rio and acme work, but those programs can be replaced without throwing away most of the OS concepts. I think you could absolutely create a Plan 9 UI that, for example, uses vi-like key bindings to jump around windows and text. Nobody's just done it yet and hardcore users are happy enough with what they have for now.
Side note: Inserting text using a debugger shouldn't be necessary. Everything really is a file, including Acme's windows and their contents (which is pretty cool).
