You make some good points, but BA received their Concordes for free. It's easy to turn a profit with a 150 million pound gift. Air France did not have it as easy. Boom's customers will not be getting their planes for free. And a 50% efficiency saving does not translate into 75% cheaper tickets. And my experience is that claimed maintenance savings are never as good as promised, and that goes doubly so for the "A" variant of any plane.