I wonder how many buyers of receivers or prepros are aware that the number of channels actually available is limited by the number of DACs included and not the total number combining extra heigth, width, and rear surrounds.

Programs to extract extra channels have been around for a long time. Yamaha pioneered the idea starting with the DSP-1 probably 15 years ago, which extracted 4 additional channels (a pair of front high and wide, and a pair of rear high and wide) from a stereo source.