AC and DC are very different!
Your computer is a DC device on the most part. Yes, it has a clock to set a beat to run things but the circuits them selves are DC. The power supply alters the AC from your outlet to DC. Within your system are crystal isolators which run at very fast frequencies which your system divides down to the time bases it needs.
3 Phase AC, is mostly a transmission system its not used by small appliances (lights, radios, TV's, shavers ...). Yes, larger appliances often use it like air conditioners & stoves.
So far the limits we are facing today is within lithography, the printing of the masking material used to create the circuits. Here the wavelength of light its self effects the making of the smaller circuits.
While I'll agree there is a limit on how small you can make things, computer processors are more than size and speed often its how the process is broken up into smaller chunks to then process in unison across multiple processors.
To help illustrate this lets look at the most powerful computers we have today Top500. Count the cores of processors these bad boys have! Now think what your own super computer you would have at home might be, how many cores do you think you might need?
Where I work we have quite a few Mac Pro systems, our most powerful is a 12 core model which is used for very large CAD drawings. Most of the day it sits idling doing nothing!
Intel has chips with 48 cores running in the lab that could be used in a Mac Pro! So a dual processor Mac Pro could have 96 cores (2 CPU chips) in a few years! Now think would would you need such a powerful system today or even in the future? Doing your taxes in a fraction of a sec. Play 96 different songs at once or watch 96 different vids across your screen? So right now we just don't have the need to go with such powerful systems for us everyday folks. Besides, these are desktop not laptop systems which is what many people want (small portible systems).
So while we are still trying to get smaller chip circuits, the need for them is still a long way away for the average person to need them.
Specialized chips is the next wave, to better process voice commands like 'Siri' being the key one for Macs. Self driving cars for accident avoidance as we have become to distracted with our cellphones! Who knows! Maybe a robot or two at home doing the chores, oops! We have some already! iRobot vacuums.