At the risk of violating my own rule about not discussing pixel size I wonder which is more likely to occur: cost reductions in the manufacture of sensors so as to allow larger sensors at less cost or improvements in imaging technology to allow improved image quality from smaller sensors.
CMOS process is a relative stable process and has been around for a long time.. We cannot expect any substantial cost reduction in sensor from the process alone, except moving from 8 inch wafer to 12 in wafer. With the same amount of physical work (mostly done with automation), a little more chemical, and a slight modification in process and you end up twice as much sensor.
As for improving the sensor performance, we should have high hope. just look at the latest sensor from Nikon( Sony). However, we have already pushed beyond the capability of the lens resolution with 18MP APS_C sensor.
As a foot note: Most other electronic device are enjoying huge cost reduction in the last 40 years due to the advance in processing technology and cost reduction in the processing itself and hence the internal device on the silicon is getting smaller and smaller and resulted in the actual silicon in each IC is getting smaller and smaller and hence it is getting cheaper and cheaper. Unfortunately. This does not appy to the DSLR snesor. The sensor size is fixed. So there is no cost reduction in shrinking internal device. Instead we end up with higher pixel count. We have moved from 2MP (APS-C) to 18MP (APS-C) in the last 15 years or so.