Quick thing before I get into my own reaction: Transpiling is normally thought of as the process of converting one language into another. Tools like Babel transpile TypeScript to JavaScript.
What's being proposed here (AFAICT) is type erasure – the generic type information would be erased during the conversion to opcodes. Python (the language) has type erasure for all its types (including its generic types), so no popular Python interpreters check that a function call's argument types match up with the function signature. PHP currently does not erase any types so the opcodes generated by PHP's interpreter include type checks for all the arguments passed to a typed function signature. Hack follows PHP's model, but erases generic types by default (though has more recently introduced the concept of reified generics) in much the same way you're proposing.
