Sure, and operating systems have been written in assmebly too. The question is whether it would be better than just sufficient if Linux were written in C++, today (ie C++17 or 20, not something old). Switching now probably wouldn't be feasible (even ignoring technical reasons, the kernel developer community is familiar with the C codebase and code standards and bought into it), but if Linux were started today, would it be a better choice?

Maybe the answer is still no and C would still be chosen, but the choice today is very different than it was when Linux was started. Of course, maybe Rust or something would be chosen today instead.