If it took 21 years to go from 32-bit to 64-bit, imagine it will take about 21^2 = 441 years to go from 64-bit to 128-bit. This is because 2^128 / 2^64 is the square of 2^64 / 2^32.
If it took 21 years to go from 32-bit to 64-bit, imagine it will take about 21^2 = 441 years to go from 64-bit to 128-bit. This is because 2^128 / 2^64 is the square of 2^64 / 2^32.