Having tried both, I think Zig is a replacement for C, while Rust is a replacement for C++.

One thing Zig has that lots of "niche" languages don't is that you can include C headers directly. This means if you want to make a game in SDL, for example, you don't need to wait until someone ports SDL to your new language. You can just include SDL.h directly and start using it. D also has this feature, by the way, but Rust requires you to generate the bindings.

Even if people move from Zig to Rust for some things or vice-versa, the strengths of Zig remain there.