Computers used to be very, very expensive. Residential telephone lines were acquired through mortgage a couple decades ago in my country so being expensive now doesn't mean expensive forever.
unlike with computers, there's some pretty obvious barriers that limit the efficiency of supersonic airplanes. your volume has a strict lower bound from the size of passengers and luggage, your engines have a lower bound since there's only so much air you can push against, and your drag has a lower bound of a perfectly smooth aerofoil that produces lift and can fit the people and luggage. Even if you take the most optimistic assumptions that didn't violate physics, your fuel burn isn't going to be reasonable.
That's a thing called a rocket. Those have different constraints (e.g. energy proportional to mgh to get out of the atmosphere, dramatically less efficient engines since you now need to carry your oxidizer, etc. It seems vaguely plausible that for very long distances (e.g. New York to China), a rocket could be more efficient than an airplane, but it does seem pretty unlikely that a rocket can be as efficient as a normal airplane.
A ballistic missile flight to go from New York to Sydney would take 40 minutes. The Concorde, travelling at Mach 2, if it had the range to do that flight, would take 7.5 hours, or 16 hours on a regular jet, if you can get a direct flight (except there currently aren't any).
Plus the added benefit of looking exactly like a ballistic missile attack on radar. :D Hope nobody with a twitchy trigger finger ever mistakes your flight for one! And also hope that no enemy will try to disguise their incapacitating strike as a scheduled flight.
No, but there are nothing I'm aware of changing the economics. Electronics were coming down in price for decades before computers reached the average house (of course when they reach the average house is debatable - on one extreme the Atari 2600 had a CPU, on the other there are still remote villages that are just adopting phones )
What’s expensive, largely, is the fuel to shove something through the air faster than the speed of sound. Short of sci-fi frictionless materials cheap enough to cover an aircraft, I don’t think you’re going to see a big breakthrough there.
