(And in some cases even retrogression -- the fastest military and passenger aircraft of all time, the Blackbird and the Concorde, were both launched around 40 years ago, and have been retired from service since. The current world record in manned aircraft speed was set in 1976!)

I don't disagree with your broader point, but I am not sure this is the best example. The Blackbird was a reconnoissance aircraft -- something that has been replaced by the superior technology of imagery satellites. In the case of the Concorde, the plane proved uneconomical, in part because many of the major urban areas that would be natural points of call did not have room to build long enough runways to accommodate its speed. Aeronautical engineering is probably capable of building a faster passenger plane, but there would be no market for it.