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.