This isn't right. JSON can also store exponential numbers (eg {"google": 1e+100}). You could decode this to an arbitrary-sized BigInt, but I can make you waste an arbitrary number of bytes in RAM if you do that. And even then, "look for a decimal point" doesn't give you enough information to tell whether the number is an integer. Eg, 1.1e+100 is an integer, and 1e-100 is not an integer.
One of JSON's biggest benefits is that you don't need to know the shape of the data when you parse. JSON's syntax tells you the type of all of its fields. Unfortunately, that stops being true with numbers as soon as double precision float isn't appropriate. If you use more digits in a JSON number, you can't decode your JSON without knowing what precision you need to decode your data.
Even javascript has this problem if you need BigInts, since there's no obvious or easy way to decode a bigint from JSON without losing precision. In the wild, I've seen bigints awkwardly embedded in a JSON string. Gross.
Putting responsibility for knowing the number precision into the language you're using to decode JSON misses the point. Everywhere else, JSON tells you the type of your data as you decode, without needing a schema. Requiring a schema for numbers is a bad design.
Ok so it allows e notation, but still the actual numerical value is unambiguous. You could parse that into a data structure that (for example) stores the mantissa and exponent as (arbitrarily large) integers. Again, that most languages try to shoehorn decimals into floats or whatever is on those languages.
In court you would be right, in practice it's on JSON. Requiring an arbitrary precision math library to correctly parse JSON is just not going to happen. The only language I know that even does this out of the box is Python with their automagic best numeric type. Even Ruby which is dynamic to a fault only gives arbitrary precision for integers and parses JSON numbers with decimals as floats.
One of JSON's biggest benefits is that you don't need to know the shape of the data when you parse. JSON's syntax tells you the type of all of its fields. Unfortunately, that stops being true with numbers as soon as double precision float isn't appropriate. If you use more digits in a JSON number, you can't decode your JSON without knowing what precision you need to decode your data.
Even javascript has this problem if you need BigInts, since there's no obvious or easy way to decode a bigint from JSON without losing precision. In the wild, I've seen bigints awkwardly embedded in a JSON string. Gross.
Putting responsibility for knowing the number precision into the language you're using to decode JSON misses the point. Everywhere else, JSON tells you the type of your data as you decode, without needing a schema. Requiring a schema for numbers is a bad design.