What I suspect is going to happen is that frontend people will just shrug and adopt the .mjs extension. Browsers don't care what extension you use as long as the mime type sent by the server indicates that the file is javascript.
I guess I should seriously subscribe to a newsletter or something.
Right now you just require a Babel runtime first, then use import for everything else not caring what type of modules they are. The acceptance test for proper module support in Node.js should simply have been that you can remove the Babel wrapper in an existing project that uses imports instead of requires :(