It doesn't really matter how much money economy brings in at the top line, if there's little to nothing to be made on the bottom line. An entire plane of economy only at market fares is not worthwhile for an airline to operate on a per flight basis. Possibly worthwhile to operate in some situations just to keep their fleet in use, but likely not.

If this weren't the case, more airlines would operate flights like that. There isn't any conspiracy between airlines to minimize plane space for economy seats. The reality is that currently, the premium classes subsidize economy and not the other way around like you originally suggested.