[Jach]

Yes, Mozilla is doing just that for Firefox, and there are lots of demo projects. (There's even a minimal kernel example I have bookmarked somewhere...) The problem is that once you accept that, you open the door for many other languages that are arguably better than C and can be introduced piecemeal just as well. Like Nim, but others too. Now you have to decide which one you want to use, evaluate their various merits, it's not just a "use this hip one that pops up on HN every week". It's easier to just keep everything in C and propose out-of-band ways of making things better. (e.g. I don't think there are any unit tests in vanilla Linux but it wouldn't surprise me if there's a bank of unit tests some other groups have written to occasionally run the latest vanilla against. People have found bugs with fuzzers. Etc.)