How did prog rock become the laughing stock...

Yep. The audience of punk was younger than that of classic prog, certainly so.

The other thing to remember is Prog didn't go away - it soldiered on through the 80s - Neo Prog wasn't called "Neo" back then, it was just called "Prog" - a lot less popular than it once was, it still sold albums.

Indeed, it did not go away at all! Sure, Marillion or IQ did not sell out stadiums, but at least Marillion were in the album charts, and some of their songs had airplay in Europe. Rush was quite a big thing in North America, I have been told. Then came the 90s and the Internet-connected prog scene we have today. Prog never died, it just shrunk back to more modest (and more healthy! - who really wants the stadium shows back? We can leave that to boy bands and girl bands!) proportions.

Also, while prog was a major current in the early '70s, it was not the leading current in terms of market sales, I think. Sure, it featured prominently in the album charts (the near total absence in the single charts has other reasons, of course), and there had been stadium shows, but bubblegum pop, glam rock, soul and the various kinds of bourgeois popular music (C&W, Schlager, chanson, etc. - depending on which country you lived in) were bigger (and they also were bigger than punk in the late '70s, of course). And people like Lester Bangs had been hating prog long before the first of the legendary three cords was even strummed! The "downfall" of prog in the late '70s probably was less deep than many of us now think.

Mostly true, however there was also a generation-change. 

While we tend to regard music in decades (the 60s, the 70s, the 80s etc.) it changes more frequently than that as illustrated by the number of different subgenres that appear within each decade that appealed to a single demographic (e.g. white teens). For example the early 60s were Beat Music (aka The British Invasion) and Rhythm & Blues followed by Psychedelic Pop and Blues Rock; the 70s had Prog Rock, Glam Rock, Blues-Rock, Heavy/Hard Rock, Pub Rock and Punk while the 80s went through New Wave, Synth-pop, New Romantic, Shoegazing/Dream Pop, Gothic Rock and many more besides (such as all the flavours of Metal that emerged in the 80s). 

The audiences for each of these came from a surprisingly narrow age-group (mainly early teens whose ages spanned roughly a 5-year period), it would be unusual for one age-group to "like" the music of the previous generation ("My brother's back at home with his Beatles and his Stones - we never got it off on that 'Revolution' stuff"). 

So when Punk arrived in late 1975 it attracted a younger audience even though the people who actually played it were somewhat older. As an 18-yo at the time I thought myself to old to be a Punk - despite being the same age as Susan "Siouxsie" Ballion and John "Vicious" Richie, a year younger than Johnny "Rotten" Lydon and David "Vanian" Left ... and much, much younger than Joe Strummer, Hugh Cornwell and Ian Dury (hence my contention that these Punk musicians listened to the same music as I did when they were younger).

The other thing to remember is Prog didn't go away - it soldiered on through the 80s - Neo Prog wasn't called "Neo" back then, it was just called "Prog" - a lot less popular than it once was, it still sold albums.

/edit: the pedant in me must comment on the "white elephant" metaphor - a white elephant is an unwanted gift that is expensive to maintain or difficult to dispose of without offending the person who gave it. Often this is confused with "the elephant in the room" which refers to a controversial subject that all can see but no one wants to mention. But that's just idle pedantry, we know what you mean ;-)

 

No wonder their same old rhetoric as that of the punk musicians, sorry but that's almost everything people always heard from them.

True, but it was also an unfortunate case of miss-booking. The Empire Pool Wembley was, as the name suggests, an arena sized swimming pool built for the 1934 Olympic games that could be converted to an ice-rink for ice shows that were popular in the 1970s. For one-off rock concerts the pool area was normally covered with boards for the audience and the stage arranged at one end (I saw Pink Floyd and Mike Oldfield concerts with this set-up). 

You are correct.

Punk? lol

Its not even considered music