I think if we start applying that rationale we could skip a lot of steps and just move everything over to YAML.

No because YAML is full of ambiguity.

JSON5 is not.

There's something to be said for being flexible in your inputs when they are non-ambiguous. Particularly when dealing with files written by hand.

There's no virtue in imposing overly strict syntax when it serves no human purpose. That's trying to alter people to fit machines, rather than altering machines to fit people.

Yeah, and we could easily say that any property value without a comma, closing bracket, or closing brace; that doesn't follow the format for numeric literals; and isn't the literal value 'true' or 'false' doesn't need to be quoted when part of a value and will implicitly be a string. This adds no ambiguity and some people are used to writing their string literals without any extra quotes, why add the extra cognitive load of having to remember to use quotes? Wouldn't it be nice to write `user: { type: admin, name: Bob }`?

This seems like exactly how we ended up with YAML-and-the-kitchen-sink.

For a spec like this, things should need a better justification than that. Everything starts at -100 points. How does this feature justify the additional complexity in the spec, to the users, and to the people trying to implement this? How does it justify increasing the opportunity for creating subtly incompatible parsers?

Or, if a goal of JSON is not to be simple and interoperable, then I fail to see how it's not just YAML but 5 or 10 years behind the curve, so we may as well skip a bunch of hassle and move to YAML and start fixing the bugs we've got there rather than creating new ones here.

I don't know what to tell you, you seem to be taking a black-or-white approach here of extremes. Like anything that isn't limited to exactly one way of expressing it is a slippery slope to YAML. It's not.

JSON5 is literally exactly JavaScript notation, which tons of people are familiar with. On the other hand, I don't know any programming languages that use strings without quotes or delimiters. (And of course, their existing in YAML is widely recognized as a major source of confusion.)

And in your example, there isn't parsing ambiguity but there would be change ambiguity when you alter abc (without quotes) to 123, because it changes its type.

I'm not aware of any change-ambiguity situations in JavaScript/JSON5 like that? In Python 5 and 5. are different types, but not in JavaScript or JSON.