To your specific points:
1. JSON5 didn't exist when JSON adoption occurred, and in any case they're pretty easy to tell apart, because JSON requires keys to be quoted. This is a non-problem. Why do you think it might matter? Not to mention that the existence of some other format that resembles JSON is hardly a reflection on JSON itself, except perhaps as a compliment to its perceived usefulness.
2. Bigint support is not a requirement that most people have. It makes no difference to adoption.
3. Escape character handling is pretty well defined in ECMA 404. Your point is so obscure I don't even know specifically what you might be referring to.