I think 64bit will last a lot more then 20 years. Its not the same as moving from 8 bit to 16 or 16 to 32, both were major leaps in terms of practicability. When you moved from 8 bit to 16 bit, you only gained 256 times the amount of working space. Sounds like a lot tho, but 256 times a tiny number was still a tiny number. When they moved from 16 bit to 32 bit, they gained 65536 times the working space. Moving from 32 bit to 64 gives you 4294967296 times the working space. Then you think of the practical size of numbers. For instance only counting to 256 is pretty useless, so is only counting to 65536, counting to 4 billion is a lot more practical. But no average task will ever need to count to 64bit limit of 18446744073709551616. I can see the 64 bit mark for memory being suprassed some day on the consumer level. But not really the need to add >64 bit integers. Floating point sure. 128 or 256 bit floating point is already useful, but we arent talking about floating point units.
Moore's law appears to be loosing steam as of late, but if you assume they can keep doubling the transister count every 18 months, and apply that to 32 doublings, thats about 50 years before the 64 bit space would be exhausted.
Humanity will likely never need a 128bit memory space tho. It would probably never make sense to move from 64 bit to 128 for instance. For integers....maybe 96 bit, duno about much more then that tho.
Think of what it would actually take to store 128 bits of memory tho. 2^128 = 3.8*10^38. If you could store each bit on a single atom(massivly more advanced then the tech we have now), and didnt need any extra atoms to actually wire the memory array and use it, you would need 3.8*10^38 atoms. Thats a lot, if we use carbon for instance, which is pretty light we get ~7,000,000,000,000 kilograms. If it was carbon in a diamond form, the densest form of carbon, it would be a cube 12-13 kilometers on each side, what is that...a cube about the size of manhattan?(if the skyline was about 20 times as tall as the tallest building, and completely fulll to the brim of diamond) Who wants to carry around their 7 trillion kilogram, manhattan sized diamond memory array?