I think it was designed as a low-level browser standard, not an authoring language. It is supposed to provide a stable, standardized target for authoring languages to compile to. Think of it as WASM for math markup. It makes sense if languages for authoring math is an unsettled design space where we need diversity and experimentation. As a browser standard, like WASM, MathML should enable people to experiment with new ideas and hopefully evolve a better authoring language than we would have got if we had tried to invent a language and standardize it at the same time.