I needed to ask about this division because I'm rather new in this community, and I created the tool which is devoted to simplify development of UI (react-ui-builder). And now I'm deciding if I need to switch generation of the source code to ES6, or wait a bit.
You can still target ES6 and have people use a transpiler to hit ES5, noting that not everybody is familiar with the tool chain(s) necessary to do that efficiently.
"Well, no true developer uses ES5"
Ignore the haters. Use what is best for the job based on a variety of criteria.