It's already being offered to a limited market with several ultra long-range biz-jets configured for corporate shuttle missions. If the fancy interiors were stripped out and a few other modifications done, some of these could probably be made to move 50 or so pax over 5000nm. Or you could probably cut something like a BBJ as a 90 seater, but it's still going to cost as much as a high-density B739 to run and is only moving half the bums in roughly the same flight time, albeit with fewer landings.
It's economy of scale that defeats this type of operation. Pilots need to be trained and paid near as much as their heavy-iron counterparts. ATC charges, airport handling etc may be (and probably should be) levied on occupancy times in the system rather than size. Also a high-tech small aeroplane consumes almost as many (sometimes more) man-hours maintenance per flight hour as a much bigger one because there is not much difference in the number of components or their complexity - only a size difference. For the beancounters, bigger is better.