I wouldn't consider a show to be illustrative of the popularity or lack there of. I'm fortunate enough to live relatively close to a large show, but the show itself draws in people from Canada as well as the PNW, so, that's a large geographic region to be drawing people from. There's I think 3 significant shows within 50 miles of me at various times during the year. It's where I wind up buying most of my stuff.I have to agree with what the majority of people have said - the hobby is in decline and will continue to decline for all manner of reasons.
I don't think you can use a "show" to illustrate the popularity of the hobby though. I may be wrong but many people attend things like that because they are "there" or "out of interest" but not necessarily because they are engaged in the hobby itself. The on thing that numbers at a show to give hope to is that "some of those people" might actually take the hobby up and help expand its survival.
You are, of course, right - not ALL model railway hobbiests (is that even a word?) belong to forums, so forums can't be used to evaluate (accurately) the level of interest in the hobby.
You got THAT right !!!
That being said, the folks I talked to last year when I attended were saying that the show itself was growing and that they'd had to expand into another building to hold the displays and vendors.
IMHO, getting people to attend shows like this is probably one of the best ways of promoting the hobby. Second really to having a friend or close relative that's serious about it. You get to see all of the things that are possible and there's usually folks there that have all sorts of helpful suggestions about how to get started.
Personally, I'm intrigued by those tiny layouts. Especially the ones that fit in a coffee table or a suitcase. And I was astonished at how good lighting has gotten since I was playing around with using tiny incandescent bulbs 20 years ago. Those campfires, welding lights and signs have gotten really impressive.