Could you operate a SST with a fuel burn about 20-25% higher than a sub-sonic? Could you shift 150 or so airframes to cover development costs? (Or alternatively, tap into an appropriately targeted NASA program, as US mfrs tend to).
Is there a market? Suppose you designed one hull, with several fuel and fitting options. One could carry 240 economy pax for 3-4 hours, at a premium of (say) 30% - Say W. Europe - E.USA. Bear in mind that there is already a market for an upgraded economy product, with more legroom. What about halving journey time? No need to travel W-E overnight!
Then another option would carry 120 premium pax up to 7-8 hours - say US-Japan or China, UK-SA, Germany-Sao Paolo. A third would sew up the first class long-haul market by carrying 30 or so first class pax around the world direct in 10-12 hours - no refuelling stops; no ATC delays or tangling with weather; arrive feeling fresh (if jet-lagged!).
OK, slightly fanciful - particularly the last one. Also lots of pilots needed, due to radiation counts. But you have to think "marketing". Someone can make you want these things!