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.