How long is a long haul that you can squeeze it in a night-park-cycle? Typically such parking occurs between 10pm and 6am, meaning 8 hours. Take away two to prepare out and inbound LH flight, that leaves 6 hours and two whooping 3 hour legs. Then back to the daily 6 to 8 milk run legs.
The timing doesn't necessarily have to fall exactly between these hours, the long haul could start earlier or arrive back later or be flown at the weekend. It gives an airline the potential for greater utilisation. Some airlines operate long haul aircraft on short haul routes to get greater use out of them instead of having them parked all day between a morning arrival and an evening departure. Others offer really low fares for flights at ungodly hours of the night when the aircraft would normally be parked, because of the unattractive timings business men and regular full fare passengers will still cough up the normal fare to leave at 8:00 am, leaving the 3:00 am flights to those who wouldn't travel unless they got a dirt cheap ticket.
In flight entertainment can easily be provided via video streaming to a passengers iPad instead of seat back units, saving on cost and weight. Lower density seating might be needed to allow for increased fuel loads so a premium cabin class could be offered to offset this.
I'm sure airline planning departments have an idea of which routes they could make it work on. Replacing an old A321 which is due for retirement with an new LR version instead of a new standard version isn't like trying to make a new fleet of A380s work.
Boeing need to come out with a single isle, 190 seat version of the B787 for short to medium haul. The B737 is essentially the same 1950s technology as the B707 but tarted up, like a granny in a mini skirt and high heals.