Wouldn't jinja templates would have sufficed for this? I'm not sure it's a good idea to create a whole new language just for getting templating capabilities to shaders.
No, I don't believe Jinja would suffice. For starters, how would you abstract out the syntax differences between, say, HLSL and WGSL? Jinja's approach seems to be taking the syntax of the target language and embedding templating into it. But with Metashade, we're replacing the different syntaxes of individual target languages with that of Python. Since this approach is largely agnostic of the target language syntax, new targets can be added in the future without rewriting target-independent, polymorphic code implementing rendering or compute techniques.