This is a really good article, and I admire the authors for the work they put in. But I can't shake the feeling that what they want to accomplish could be done

This is a really good article, and I admire the authors for the work they put in. But I can't shake the feeling that what they want to accomplish could be done with a library rather than a language extension. Can anyone explain why their issues couldn't be resolved with a particularly clever mix of traits and macros?