Perhaps you're correct about C++, but I was more referring to the Zig vs. Rust situation.
Since the Zig tool chain can also compile C, and Zig can use C headers without translation, the cases are more similar than one might otherwise suspect.
But of course it would be less of an upgrade, and the Zig parts would have to stay clearly segregated.
There are plenty of loadable modules in C++ for Linux and BSDs, and plugins for Postgres, in places where there is no expectation of upstreaming them.
Zig is in a similar boat.