WebGPU has some surprising performance problems (although I only checked Google's Dawn library, not Rust's wgpu), and the amount of code that's pulled into the project is massive. A well-made Metal renderer which only implements the needed features will easily be 100x smaller (in terms of linecount) and most likely faster.

There is also the issue that it is designed with JavaScript and browser sandbox in mind, thus the wrong abstraction level for native graphics middleware.

I am still curious how much uptake WebGPU will end up having on Android, or if Java/Kotlin folks will keep targeting OpenGL ES.