This is a topic I love
I guess it's because if you crack this then you've cracked the entire collectibles market. I don't view SW as a source of income (exactly the opposite, actually) but I do buy and sell general collectibles: from medical instruments to board games.
As you quite rightly say, Scott, it's the nostalgia kick and Vintage SW is only really relevant to our generation. Sure you'll get the outliers of collectors in their teens and in their 60s, but the core is as old as the films, give or take 5 years. Yeah there is bound to be a SW market forever, but this old stuff only really means something to us. Parallels can be drawn with any other collectibles market you care to name. Antiques are a little different and have a market of their own. This is linked to tastes and fashion. Remember briwn Victorian furniture? A decent dining table and chairs would have cost you £1,500 10-15 years ago. Now you're talking £300. Same with all sorts of antique stuff. The market there is cyclical.
I'm a firm believer that when we're dead this stuff will be worth 20% of what it is now. It's one reason I can't bring myself to spend thousands on MOCs anymore but really hunt for bargains. No, I'm not buying as an investment but if I'm spending my kid's inheritance then I really don't want to lumber them her with a load of toys that she has zero interest in and are a pain to sell. I collect for me but I don't want it to be selfish, if you see what I mean.
We'll see collectors come and go, as with any hobby. We'll see prices rise and fall, as with any hobby. We'll the manufacturers reinventing the wheel (again and again and again in Hasbro's case) as with every hobby. But I don't think we'll see a solid decline in the market for 20 years. That's when the grown up us decide we need to really start thinking about our kids and grand kids. That's when we downsize (not give up) and sell off stuff. That's when supply outstrips demand.
Still, if the sell-off is linked to retirement age then the sell off won't occur until until our bloody 90s at this rate