Unless I'm missing something, a major downside of this is that scale types are not explicit, and that is very, very, very bad IMO. It will lead to numerous cases of poor interoperability as scalars are interpreted differently.

That is, in JSON, at least I know "true" represents the string "true", and true represents the boolean value true, and "1.25" is a string and 1.25 is a number. If anything, JSON would be greatly improved by having a specified datetime type, and more guaranteed semantics around numbers (e.g. integers vs. decimals vs floats for example).

XML obviously suffers the same issue, and I think it is much worse for not having types (though highlights the difference between structured "documents" vs. object serialization formats).