Ad 2) It isn't just about IE anymore as more and more older machines become outphased for current OSes. E.g., for a 10 years old Mac Pro, which is still a viable machine, Firefox is currently the only evergreen option which is still receiving updates. (Safari is largely OS-dependent, as is WebKit, and Chrome/Chromium cancels support as soon as the OS is phased out.)
1. Node.js greatly benefits from these features. Transpiling is not as prevalent there
2. Some people do exclusively target "evergreen" browsers and don't care about IE support
3. Those that do differential bundling (different bundles per browsers) can see quite a performance boost by not transpiling on newer browsers