I have played with Ada but not done anything “real” with it. I think I’d be ok with using it. It seems better than C in most regards. I haven’t really looked into Rust but from what I can gather, its main innovation is the borrow checker, and Ada might get something like that too (influenced by Rust).

I don’t understand why Linux is so huge and complicaed anyway. At least on servers, most Linux kernels are running under hypervisors that abstract away the hardware. So what else is going on in there? Linux is at least 10x as much code as BSD kernels from back in the day (idk about now). It might be feasible to write a usable Posix kernel as a hypervisor guest in a garbage collected language. But, I haven’t looked into this very much.

Here’s an ok overview of Ada: http://cowlark.com/2014-04-27-ada/index.html