The sonic cruiser was killed by september 11.
Fuel costs have been more or less flat for the last 30 years or so and will stay that way within a trading band.
What has gotten out of control (from the airline's point of view) is the crew costs. With the United pay raises that were making their way through the industry the value of speed (even an incremental increase) became quite large to the airlines as they could get more "work" ( eg.miles flown) in an hour out of a crew by going faster. So a fifteen percent increase in speed would be a 15 percent productivity gain for the airlines. That was BIG numbers and worth far more than a 15 percent increase in fuel burn.
Instead the airlines will get their productivity gains in the bankruptcy court instead, without having to buy ANY expensive aircraft, boeing or Airbus.
Cheers
Wino