The one thing that occurred to me in reading the responses of this thread is that a lot of the characteristics of early 70s prog have been absorbed by other genres. Not just long, non-traditional songs, or incorporating new technology into music, either. You've got 90s acts like Bjork, Radiohead, Sigur Ros, and many others who really took experimental elements of music to a new level, but would never be considered prog. So as the initial spirit of prog carries forward to new and unexpected places, prog continuously becomes further defined by certain tropes that were common of the 70s band and the retro bands of today. A similar thing happened to jazz as the old guard became increasingly academic and disconnected with the new blood. Any genre more concerned with specific traditional or aesthetics is probably gonna go in and out of phases, while there will always be something new carrying the torch that isn't necessarily accepting or even acknowledged by the old guard.