If you're using a 64 bit integers because you've got some super high precision math you need to do over an enormous space of addressable numbers, like maybe you're firing unguided kinetic energy weapons at enemies on other planets... sure, use big numbers. I'm sure you've got some clever libraries able to do such things reliably, and I won't question why you're using json as your serialization format.
If you're using 64 bit numbers as a high cardinality identity that can be randomly generated without concern for collision (like a MAC address with more noise) -- well, that's an identity and doesn't need to have math applied to it. For example: "What's the mean IP address that's connected to cloudflare in the last 10 minutes" or "what's the sum of every mac address in this subnet?" are both nonsense properties because these "numbers" are identities not numbers, and using a data type that treats them as numbers invites surprising, sometimes unpleasantly so, results.
Of course, because these are computers, all strings are ultimately numbers but their numberness is without real meaning.