Supersonic flight adds a new source of drag, called wave drag
, which comes on top of all other drag. It depends on the slenderness of the plane, but can easily double the total amount of drag. Optimizing the design for less pronounced shock waves will increase drag yet again, so fuel consumption per mile flown will make the cost of supersonic travel prohibitive. After all, the travel speeds of modern airliners (Mach 0.78 to 0.855) is typically a bit lower than the design speed of early jet airliners like the Convair 990 (Mach 0.87) or the Vickers VC-10 (Mach 0.89). That was half a century ago!
But there is a pocket of aviation where progress has been made in flight speed: Business jets! While the first generation flew more slowly than airliners (Lockheed JetStar, Lear Jet 25), the latest designs are quite a bit faster (Cessna Citation X, Gulfstream V) at up to Mach 0.935. Why? There is a peeing contest going on among their owners who is the fastest. A very small segment of mankind is licking their fingers at a new chance for showing off. A supersonic business jet would be a sure sell to this crowd, even if the operating cost per mile doubles.
Well, see it this way: This is a chance for the other 99.9% of mankind to lower the Gini coefficient a bit.