This take is disingenuous and horribly uninformed and very shortsighted.
Not every parser out there is JavaScript eval(). Some people actually had to write and maintain a parser in whatever programming language they have to work with.
In the past I wrote a JSON parser for a low-level language, and back then I had to write the letter and parser by hand. Each change to accepted JSON values meant I had to add at least one terminal token, update the grammar to include it, and update the data model to support yet another type.
Adding comments would either require adding a terminal token to be ignored, or rewrite the data model to support positional items that cursors/iterators could or could not skip.
These are not 13LoC changes. At all.
And if I happen to be consuming an API I don't control which for God knows what reason decided to move to json5, I would either have to rush a parser to production or remain down.