... it ought to be possible to design a nice large sensor without too many megapixels, and keep the price low enough that the sales numbers increase ...
I will say it again: that cost of making large sensors is determined almost entirely by their size, not by the pixel count (cell size). Combine that with the fact that having fewer, larger pixels has no demonstrable advantage in IQ when compared fairly (at equal print size or such with downsampling if appropriate, and with the same generation of sensor technology), and there is not much reason to design new sensors of lower resolution than exiting ones, except to help with frame rates and processing speed.
If anything, designing and having two sensors of the same size but different cell size only increases costs (more R&D work and lower economies of scale in fabrication and stock control), so this only makes sense during the transition to a new higher resolution cell design.
Since Kodak has now launched a 44x33mm 40MP sensor with its new 6 micron cell design alongside its 49x37mm 50MP model, I expect the older 6.8 micron Kodak sensors to be discontinued sooner or later, as the 9 micron ones already have been.
P. S. MF sensor photosites are huge by the standards of modern IC fab. technology, so the issue of space wasted between cells is minor. Worry about that with sub 2 micron cells of compact digicams maybe. DSLR sensors do no even need to go close to using the smallest feature size for fab. due to their generous cell size.
