amd64 isn't going away any time soon. Intel is still the largest player here. If I were Intel right now, I'd be trying to come up with the next iteration of x86, probably 256 bit.
but, why? do we need more than 16 exabytes of memory?
Joking aside, we very well may one day have more memory available that 64 address lines can handle. RISC-V’s spec does have a stub chapter for “RV128”