I mean, you don't have to support those? You still would need something on the other end to produce that type of datatype, which can be documented that it will never happen: you're making an interface anyways. The problem is if you literally don't have the option to represent common datatypes it will be a problem, not a hypothetical one just because the encoding layer can support it. Those are different problems.
And JSON, technically, allows use of unlimited-precision fractions, but also allows implementations to set arbitrary limits (it actually does, you're not required to parse JSON numbers as doubles). So the situation is not really different from CBOR, isn't it? Just™ make both sides to agree to stick to some common subset (e.g. integers-in-int64_t-range-only for some fields) and you're done; no need to support double-precision floating point numbers.
Huh, I went and referenced the ECMA JSON spec and you're right that it treats numbers only as sequences of digits which would make these effectively the same problem
