It seems to me that if Concorde "failed," it's only because it did not usher in an era of supersonic flight. The significance of airplanes like the DC-3 and the 707 isn't so much that they were great airplanes (though they may have been), but that they were copied and improved upon. That didn't happen with Concorde, and I think the only reason can be that the time just hadn't come for supersonic passenger flight. I guess we'll see if it ever does.