But writing an entirely new, bespoke high-level shader programming language is more robust? And robust in what way? The SPIR-V format is vastly easier to parse than a textual language.

And WGSL will still bounce through HLSL for DirectX because DXIL is an awful, undocumented mess of ancient LLVM-IR with a giant pile of bolt-on special semantics. Directly authoring DXIL is awful.