12nm might be a technical limit - but if those 12nm transistors cost more than a 28nm transistor then nobody wants them.
The main reason for going small is cost - Silicon costs you per sq in - smaller transistors are supposed to buy you more hardware for the same money. If that stops there is less reason to go small.
To clarify, there are still advantages to 12nm, in that the speed of light allows the 12nm transistor chip to be faster (all other things being equal) than the 28nm chip. However, there are limited uses for that speed at this point, which bring into focus cost/performance.
Further, part of bringing costs down involves efficiencies of scale. If you don't have the scale, the price will limit the output. It might be back to the supercomputer world for those who need that particular extra bit of speed.
The main reason for going small is cost - Silicon costs you per sq in - smaller transistors are supposed to buy you more hardware for the same money. If that stops there is less reason to go small.