Rust needs to stay out of base, if anyone wants programs in Rust, they need to make a separate ports tree which is in Rust, and uses FreeBSD as the base system. Or they can use Redox or another operating system, then use existing C programs outside of their base system to make up for what they don't have.
If anything goes into the base, it needs to be a C based modern safe language, and able to compile C. Zig or a successor is an acceptable candidate to go into base. This way, there will be one compiler, which compiles all needed in base that's both C and Zig. It's time for a programming language based on C, which is safe and replaces the functions of both C and C++, and can also use C libraries as is and perhaps C++ libraries. A fork would be a safer option.
As long as C is around, FreeBSD will survive. Then, it would still survive in maintenance mode after C is replaced, which would be after a long time. The successor form of FreeBSD built directly off of it will be around for a long time as long as that next language stays around. I think it won't be Zig, but will be something based off of Zig with its lessons and will include Zig founders into a newer language.