Ben Thompson: We never really knew how low the [flight] price could get, because there was no competition.
Jon Ostrower: Exactly. Yeah, exactly. So, if you want to justify the business case, what ends up happening is you have to have airlines on the same route. If you put two Sonic Cruisers on the same route, what ends up happening is they end up competing against each other to get the premium passenger paying for speed. And then, “Oh, by the way, all the wide body aircraft, slow pokey wide bodies, are going even lower to keep those aircraft running.” And so what Boeing found was that you take a city pair like New York to London and you actually take money out of it by introducing these airplanes, it’s actually a net loser.
—By Ben Thompson stratechery

Sonic Cruisers can fly mach 0.98. This interview explains why supersonic flight is great for consumers and terrible for airlines.