Say what you will about TempleOS, but HolyC is actually brilliant. The same language is used for the OS, programs, and shell, the entire OS is JIT-compiled (modify the source and reboot to make changes), and it's just generally a really different way of doing things. You can also put vector graphics in source code.
I agree with you completely. The idea of a single language to rule the entire system is brilliant, and is something I wish others would consider putting their attention on - i.e. distro builders who compose a "Lua-only" stack, or a "Python-only" stack, and so on.. well, such things exist in Dockerland easily enough these days, but one wonder what it would be like if the 'modern desktop OSes' had this hit-one-key-and-access-code-anywhere mentality .. it has so much appeal over the current worm bucket o' mystery that is the modern OS 'distribution'/'package'/'bundle'/'module collection' ..
Smalltalk-80 and the resulting modern implementations of it follow this idea. Typically, the IDE/development tools come with the runtime environment and these allow the browse the system's code as well as inspect existing instances of object. There's also a primitive form of a built-in VCS allowing the viewing of earlier method versions and reverting to them. Finally, the underlying VM is written in a mixture of C and either Smalltalk or something resembling it.
It could even be C with something like the tiny c compiler. It can compile C at such ridiculous speeds it makes compile times practically irrelevant. I've never even been able to throw enough at it to know how fast it actually runs on a modern computer. SQLite and Lua both print a timing of 0.001 seconds (and that is both compiling and linking I think).
Emacs has what you called the hit-one-key-and-access-code-anywhere mentality, and I think part of the reason Emacs users tend to use it for much more than just editing text is exactly the appeal of an OS with that mentality.
This is what makes http://www.redox-os.org/ an interesting project. Full stack Rust OS, which is probably the most advanced language you can use right now that can also bootstrap itself from ASM.
