> Extending the syntax is undoing all the communal work
JSON5 takes stuff from ES5.1 and adds it to JSON, and ES5 is backwards-compatible with older JS. This means it's fairly trivial to convert JSON5 to JSON (literally 13 lines: https://gist.github.com/iddan/3d34b12f6b22c30a8a07c149b3175e...). And ES5.1 itself was the product of communal work.
> encroaching on grounds that YAML and other languages already cover.
I don't know that we should really start dividing features into nice little boxes so no one steps on each other's toes. Trying different solutions for the same problem can eventually bring us more optimal solutions than if we just sat around twiddling our thumbs because 'it's been done'. It's not like there's an army knocking on your door going "switch to JSON5 or we'll kick your teeth in." People can stick with what they want to do on the merits, but I don't see that as a reason to shut down others' work.