All-business aircraft are a rare breed relative to the APEX market. It's hard to entice status flyers to remain loyal if premium upgrades require switching to a completely different flight. In my view there is nothing wrong with the BC experience on the B787 so I see no need to spend a fortune removing it.
That depends on how you handle the scheduling. If you're sliding folks around by a few hours and moving them halfway down the terminal, that's one thing (visions of the ending of
Airplane come to mind...). If the flights board simultaneously, leave within five minutes of one another, and use neighboring gates (but the SST gets to the destination a few hours faster), that feels like an entirely different ballgame.
As the main "target" market would seem to be the daylight East Coast-Europe markets, on the present timetables an easy example is that you could have an SST "shadow" Virgin Atlantic's morning flight out of JFK (depart JFK 0815 NYC time, would arrive LHR 1710 London time if you go by the old Concorde schedule) and then "shadow" the return flight (depart LHR 1850 London time, arrive JFK 1745 NYC time). Presuming a 787 vs a 50-seater SST, you could probably begin boarding simultaneously (or even start the SST after the 787) and still get the SST "out the door" faster. The question there would be turn times, but I think that timetable (which is not far off of BA's pre-accident Concorde schedule...you could presumably slide the relevant flights by 10-20 minutes if needed to get the turn time right) would generally work. The reverse schedule would be trickier...but you could probably have the LHR departure tag to one of the first flights in the morning and then the JFK departure to one of the
last departures at night (an SST with a 2130 departure JFK time would actually arrive at about the same time as the 1830 flight out of JFK...the idea of an "overnight" TATL flight only taking four hours but still avoiding graveyard hours is kind-of surreal).
Now, my guess (FWIW) is that you don't totally
eliminate the front cabin on the 787...but you
can reduce its size.
(Notably, if I'm not mistaken then it
looks like you could probably run a round-trip on the SST with a single crew and no hotel needed versus the 787 needing two crews and a hotel night to work.)
If Boom's range pans out, 4250 NM puts a number of city pairs within reach (if you can get the sonic boom to go off in the right place, at least). SEA-TYO comes to mind, as does (surreally) SEA/YVR-LHR (though this might be impeded by rules on where the plane can go boom...an uncomfortable choice of words if ever I saw one). Does anyone know if Canada has a CONUS-style ban on sonic booms? I wonder because much of the great circle route up there is
extremely rural.