Personally, I don't think C++ would give a similar improvement to Rust or a future competitor to Rust. C++ has a severe useability problem. Many companies have to really restrict what subset of C++ is used based on use cases. I don't know what contributing to the linux kernel is like, but I imagine having readable code really helps. Paring down C++ to the subset that gives measurable improvements while also being readable seems daunting. Where do you draw the lines? Will concepts be allowed, what about template heavy code and SFINAE? If you go down the C with classes route (which doesn't provide too much advantage), it might be easier to for existing devs to work on it, but if you go too far the other way, you could alienate a lot of people who contribute to the kernel already and don't have the time or will to understand the complex template deduction system of C++.

Rust seems to be much more opinionated on how to do certain tasks, which might make it an easier to use for the kernel because there will be one way to do certain tasks. That's my read on things at least.