So first of all, Apple invented a new language to replace Objective-C (they also added generics to Objective-C), you might have heard of it. Secondly, some kernels have already been written or partially written in C++ (the NT kernel for example). Of course, not many people are writing kernels, but those writing new kernels (fuschia, redox) are being written in either a mix of languages or a completely new language. Otherwise, older kernels like Linux were written decades ago in a very different landscape when other languages made little to no sense. Things have changed.

So your comment is kind of unsubstantiated. And as I said, things have changed, and things will change. We can either do as you imply and keep using C and languages without "sum types" and "real generics", and keep creating security vulnerabilities and broken software, moving at a snails pace because we have poor methods of reuse, or we can aspire to build better tools to build better software. I am squarely in the latter camp, I think we should aspire to keep evolving.