Prog rock is art rock.
Discussion over.
Yeah.
The fusion of non rock music with rock to make new music. Sophisticated classical harmonies, is the favourite basing the music around symphonic progressive rock but not limiting progressive rock.
Progressive rock is music that moves beyond roots music.and beyond rock where rock has incorporated blues or folk for example but not made it more harmonically sophisticated. KC for instance eschew blues and initially used classical motifs to define their music, along with some jazz elements. Later avant-garde experiments made the laboratory that is Crim into protein music (by which I mean no production sugar or carbs to make it "palatable" i.e. the audio compromise that makes something fit public taste.
The other side is the necessary audience, some either go with what they know and hate it when the band starts playing different tunes (Yes, Genesis) or love the more out there elements (e.g. Magma, Can).
IMHO the arguments that are born from absolute extremes e.g. NO PF without Syd, Or Roger, or Rick are involved in personalities not progression. Progression implies change and many people don't like that progressive musc fans being people as well. Just look at the results non prog artist Neil Young has when he plays the unexpected.
Or the controversial pop element. Oddly perhaps but quality pop (Mike Oldfield's is perfectly suited to progressive rock. Pop eschews roots music, is more correct in academic terms (Abba) and allows classical and sophisticated elements. Of course there is the mindless dreck which has none or little of these qualities; hideous creations that are about...
Cultural identity....
....whereas prog rock IS about music. No choice really. There is no fashion element, unlike metal punk hip hop reggae usw etc. So this is where arguments get confused. Pop culture is based entirely around music that has meaning for the moment then that moment moves on. e.g. grunge. I suppose the closest would be the hippy fashions of the 60s and 70s when the golden era began and ended and then the real test for progress began; how would old attitude react to new formats, ideas etc. (Predictably rather negatively e.g. Asia who really made symphonic prog rock in an era that was only the start of our very common ADD).
If people want to listen to great music there is Tchaikovsky, Miles, Beethoven. Zeppelin, Coltrane, Crimson and Zappa. But prog rock exists in the music world not pop culture - though some times it tries (metal) but alas, has to settle for being about music rather than the easy to market stuff like fashion. How irritating. No wonder no one likes it. No wonder P Tree get moved from alt rock to mainstream in my city's music store.
Prog rock is about musical adventure.
Thanks for listening.