The extension does not require any support from the GL drivers, so that's why I've submitted the proposal only to the WebGL registry at this point. It can be implemented purely as a transformation of the GLSL code. If there's a strong reason for adding it also as a native extension, I have nothing against that, of course.
If it is just a GLSL transformation the obvious question is why not just require WebGL implementations to do these transformations whenever mediump and lowp are used in a shader? That seems much easier for developers than wrangling debug versions of shaders and an extension. The only downsides I can see are
- Some apps might break but those apps would already be broken on mobile
- Some shaders might run a little slower on desktop than they did previously.