Not everyone thinks of Zig as a "no downsides and all upsides" C-replacement. First, a lot of people will take issue with it still being in beta and it being unknown how many more years will it take to reach 1.0. There are a bunch of C-replacements, or at least viable alternative languages out there. Both old and new. With more "C-killers" likely to pop-up in the not so distant future.

There are also a lot of people, after doing their Zig language reviews, that don't like it. Muratori (Handmade Hero) won't touch it and there was a recent article that's been covered on here and other sites, where the person explained why they stopped using it (linked below).

https://strongly-typed-thoughts.net/blog/zig-2025 (Zig; what I think after months of using it)