Using a single signed 128-bit integer does not solve this. What happens when you have to add two of these together? Do you need a 129-bit signed integer? What about if you need to add 64 of these together; do you need a 134-bit signed integer? The point is that 128 bits is well large enough to hold the sum of quadrillions of transactions in Zimbabwean dollars. Nobody is switching to a 129-bit number to hold the results of sums; they are explicitly not handling the case where any account is anywhere close to 10^38 of the base unit.