If it costs in the same ballpark per seat as a long range private jet, then for nearly all city pairs under roughly 5000km it's difficult to find an economic justification.

Especially if the plane needs long runways to takeoff.

Due to the travel times others have touched on to the major airports in nearly all big cities.

Lighter private jets have a huge advantage in travel time of being able to take off from the closest small airports.

If it's a really long route, like Sydney to London, then it would start making sense. But this could not support the thousands of engines needed to break even on R&D costs.