The fact that a JSON parser is the sort of ecological niche where you naturally get a single best library which does basically what everyone wants it to do with minimal dependencies is exactly the argument for putting it in the stdlib, though.
That would require moving the entirety of serde and its traits (or equivalent) into the standard library. Considering how long it takes to stabilize basic things in the stdlib, I think that’s a terrible idea.
IMO Rust has the best tradeoff between stdlib and ecosystem libraries I’ve ever seen in a (compiled) language, and that includes serde/serde_json.
