I'm imagining they don't want to deal with transpiling for maximum browser compatibility, or with bundling; there are existing tools that do those things really

I'm imagining they don't want to deal with transpiling for maximum browser compatibility, or with bundling; there are existing tools that do those things really well and you can always feed the output of this through those if you want those qualities Targeting ES6 will yield simpler, more readable compilation output (which is a stated goal of the project). It can also preserve module boundaries while keeping client-side support, which is not something you could do with CommonJS. And they mention other things like async/await support, which would be a huge pain to try and implement yourself instead of leaning on the target language