I imagine that Zig has a lot more focus on "I can't use X in my environment" types of situation. It seems that for many such situations it might be a better fit than Rust.
There is, for practical purposes, noplace where one can't use C++, except where gatekeepers exercise power keep it out. Typically it would be only a day's work to get those building with a C++ compiler, whence they could begin modernizing.
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.
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.