Nobody is using all these languages at the same time, anyone who targets all these platforms already has some mechanism to turn HLSL or GLSL (or whatever) into what the target platform needs. That's why Slang will have difficulty finding adoption, it's the N+1th solution to a problem that already has been solved N times.
I see your point, but one issue is that you can't go from HLSL or GLSL to WGSL for compute work, as the way atomics are typed is incompatible[1]. That's a primary reason we (currently) use WGSL, because going from that to legacy shader languages does work. But then you have all the limitations of WGSL.
Slang had to make a change to its type system[2] to make this work. But they did it, and as a result Slang is appealing as an "apex" language - you can translate from that to anything, and also get advanced features lacking in WebGPU.
For a greenfield project, perhaps that makes it a good choice, but using it to replace established systems needs more arguments than "it's better". Is it really better than a existing, bespoke solution? Keep in mind that the shader language is just one part of targeting a graphics API, you still need a lot of boilerplate surrounding it and Slang doesn't (and probably shouldn't) solve that problem.
RetroArch uses all of them. HLSL, GLSL, Vulkan, Slang, Cg as well as proprietary shader formats used on consoles, as some platforms only support specific formats, so they often convert between all of them to offer as many shaders as possible across all combinations of platforms.
But one of their big contributors also worked on Vulkan and Slang/SPIR-V itself if I recall correctly (and works at ARM on GPU drivers), so there's that.
Retroarch can ingest GLSL, Cg, and its own extension of GLSL it calls "Slang shaders." Cg is deprecated. It uses SPIRV-Cross, the de facto solution to transliterate to other APIs. The "slang" in this article is an entirely different "slang".
The guy who created Retroarch, Themaister, seems to work for Valve now, and continues to work on SPIRV-Cross among other things, making gaming on Linux actually viable.
