My main problem with SPIR-V specialization today is that it doesn't affect the shader interface. For WebGPU, we don't want to validate presence of resources that the user doesn't need, we don't want to allocate registers for them in our pipeline layouts, etc. I think having this ability is critical.

Specialization constants don’t seem to cut it for all the use cases. There’s variation beyond plugging in constants. Also some frameworks and engines give their clients the ability to provide shader snippets at runtime.

The answer to shader snippets in the SPIR-V world would be linking. I don't think that's well developed in the ecosystem if at all, but it's a perfectly natural extension.