Brian, you are probably quite correct in your assumption that in general more models (all modelling disciplines ) are ''assembled'' now rather than being built from raw materials, and that seems to be mostly due to younger folks desire for speedy gratification, though in very many cases they do not have older relatives living near them that often were the source for we older types inspiration and guidance. Modern schooling and the workplace do not help because they no longer instill the notion anyone can attempt complex activities without 'courses, ticks in boxes, safety assessment' and so on.
Surely though, the balance on this forum is not as one sided as your weekend browsing results suggest , there are so many scratch builds going on in the many sub-divisions, that I would have though the balance is around 50/50. For my own part, all my efforts are from plans or are scratch-builds, usually out of the head/scribble pad efforts, and I have 2 of these progressing (albeit slowly) within Mayhem.
I don't think model building has died, but you are right, there is no doubt that it's in decline, just take a look at 1950's Aeromodellers or model car or boat magazines, back in those days there wasn't an Idiotlantern in the corner of everyone's room that folk squandered the time staring at.
Trevor