Read somewhere, and don't hold me to the numbers, that Boeing "bet the farm" when they built the 707 and had to sell like 300 air frames just to cover R&D costs. Then again 1000 air frames for the 747.
The Orient Express, that takes off like an airliner and then goes to ramjets and skips across the earths atmosphere at supersonic speeds was deemed to be doomed. R/D costs vs. number of aircraft to saturate need. You can't benifit from it's speed say NYC to LAX because you have to orbit the earth before you land. City pairs that work requires only 9 aircraft, 10 with a spare.
The A380 doesn't fill the gap.