You could decode all of them as BigInt, but then you would be accepting a large overhead for the much greater number of constants like 1 or 42 where it's unnecessary. Everywhere you pass that JSON data would need to be updated to operate in terms of BigInt instead of number.

If you only decoded big ones as BigInt, you would have the same problem of updating the code but now you have 2 codepaths at every callsite!

It's perhaps a defect in the JSON spec that it doesn't have a provision to support BigInt, but it's not really clear what should happen!