Yes, the source-map support in browsers really needs to improve. NodeJS is another matter though as, in my experience, it maps seamlessly.

I think Coffeescript, TypeScript and Babel (ES6 and JSX) are completely different beasts to Elm though. I can look at the compiled output of these languages and debug that, I don't really need source maps (though they would be nice).

I guess the idea for Elm is, you won't need to debug as you won't get runtime errors.