MIR isn't really a language of its own: it's a human-readable serialization of Rust compiler's lower-level AST, an artifact of the language's complexity. It's native to the compiler, so it doesn't need to be parsed, validated and compiled. So it isn't really relevant in this context.

Rust aside, I'm fine with using C as a target language, as it's well known and has all sorts of tooling, but inventing a whole new language with a custom syntax just to use a codegen is overkill, especially when there are more than a dozen projects that use SSA-based IR to great effect (from LLVM and golang to game console emulators).