> It's a much smaller step from 2 to 10 than from 1 to 2.
But here, you're already starting with 2: C and assembly. Besides inline assembly, a small but very important part of the Linux kernel is written in assembly on every architecture: the system call entry point (entry.S) and the kernel entry point (head.S). And if you consider each architecture's assembly as a separate language, it's more like 10 languages than 2 languages. I'm always impressed whenever I see changes to for instance signal handling or thread flags which touch not only the common code in C, but also the entry point assembly code for each one of the many architectures Linux supports; whoever does these changes need to not only know the assembly language for all these architectures, but also have at hand all the corresponding tooling and emulators to compile and test the changes.
You do have a point, however (as you noted) the lowest-level bits of an OS kernel are practically impossible to build (and subsequently, maintain) without precise control over the machine code; you can't even start a hobby OS kernel project without relying on assembly. It's a part of the deal; a pure-assembly kernel is more feasible than one without any. You also (as you pointed out) still have to be mindful about the C-asm boundary; the integration doesn't come free.
The story here is pretty different: integrate a new, high-level language into a 30 year old, 30mil SLOC, production code base, that billions of people rely on every day, AND actually extract some value from that work.
But here, you're already starting with 2: C and assembly. Besides inline assembly, a small but very important part of the Linux kernel is written in assembly on every architecture: the system call entry point (entry.S) and the kernel entry point (head.S). And if you consider each architecture's assembly as a separate language, it's more like 10 languages than 2 languages. I'm always impressed whenever I see changes to for instance signal handling or thread flags which touch not only the common code in C, but also the entry point assembly code for each one of the many architectures Linux supports; whoever does these changes need to not only know the assembly language for all these architectures, but also have at hand all the corresponding tooling and emulators to compile and test the changes.