I started reading this thread, got half way through and skipped to the end. Sorry if this was all mentioned already in the last 4 pages I didn't read.... (yet!)
At the risk of sounding like a conspiracy theorist I'm going to start by saying that our culture has changed DRASTICALLY in the last 20 years. The advent of computers and the internet has created a sudo "Matrix" kind of lifestyle that a lot of people seem to enjoy. Not Only are people enjoying "living" in the online world (be it facebook, scubaboard or some MMO / video game) but corporations are pouring billions of dollars into campains that do their best to make us want to spend ALL our time there (every second you spend online, you're making somebody money) Corporations are extremely good at making us want to stay "jacked in"
This has created a culture of people who go online for entertainment. Do you all honestly think that the dirt bike, quad, race car, ski mountains, (the list of young people sports goes on) AREN'T having the EXACT same problem as scuba is having with gaining new young people's attention? Its not even just the sports, but the arts as well. Singing, dancing, painting, sculpting, musical instruments... these are all areas that are seeing exponential loss of attendance from anyone that isn't being pushed into it by their parents.
The simple reason that young people's attendance is so low for scuba diving is because they'd rather be entertained online.
Don't get me wrong, there are still plenty of young people that go out and are active outside the computer/xbox, but for every one you see out there (i'm pulling a number out of my butt here) there are probablly 1000 more sitting on their rear gaming or chatting or facebooking, or otherwise online.
I've recently turned old. (35 yesterday ;D) and I've grown up in this new, online culture and I have to say that of ALL the guys I know, I'm the ONLY one that actually gets off the couch and does stuff (I actually rarely get on the couch, but thats beside the point)
Anyway. Forget the money problem. Even when I was just a kid, I could come up with a grand if I really wanted something. A grand is nothing. And the fact remains that if you aren't "close" to a dive site, or live in an area where there is easy opportunity to dive, then you are going to have to go "out of your way" to do it, and thus, it will be less attractive and ALWAYS more expensive. However! If you DO live close to a dive site, the cost is SIGNIFICANTLY lower.
Lets assume that ALL the dive sites we go to, where we notice the complete lack of 18-28 year olds does in fact HAVE a population of 18-28 year olds in the direct vicinity, that HAVE the opportunity to dive relatively inexpensively (due to the close proximity to easy diving) but STILL aren't out there diving, We have to assume that these people are CHOOSING to do something else, NOT because of cost, but because they'd rather be doing something else. (and I've already expressed what I think that something else is) Now, that explanation firmly in mind, I think we should all be able to agree that it really ISN'T the money or the cost that is preventing these people from deciding to dive.
Now ALL THAT SAID... I gotta go. I'm 60% way through the new GTA 5 game, and there is some pretty cool virtual diving in it!