Umm, but Wayland and PulseAudio do have accessible interfaces from the file system... Wayland even requires it, as opposed to X11 which could work over TCP socket as well. No idea about systemd though (I think it uses dbus?)
These are only sockets through, through which you have to multiplex-serialize all the commands.
In the Plan9 model, each and everything is represented by an individual file system entry. In Rio (Plan9's windowing system) every window is represented by a directory in a 9p tree, and you draw to it by opening a framebuffer file inside the window directory and manipulating that.
Furthermore, terminals know about their windows, and can hand it over to whatever is started inside them, which essentially makes it possible to open windows "inside" a shell environment.
It's kind of a unified screen/tmux environment, and you can stack, mix and match how you desire.
Sure, but that's the Plan 9 philosophy. UNIX doesn't even have /proc/$pid/fd/ - there's no way to refer to another process's stdio unless you have the fd passed to you, and stdio is kind of the basis of UNIX. So by UNIX's definition of "everything is a file," we're still there.
As mentioned earlier, Plan9 is what UNIX research edition 8 grew into (as I understand it), it's possibly one of the best examples of the UNIX philosophy.
It's important to note that UNIX the actual implementation is a moving target because of the many versions, just because the designers didn't see something could be achieved in UNIX (resources, time, effort, etc.) or just plain didn't think about it, doesn't mean that it doesn't embody the UNIX philosophy :)
