Sadly, I think this is the natural course for many things that come to be considered collectible. There are overall trends (like the fixie craze and the hipster desire for old school cred, the Japanese interest in "exotic" European vintage bikes, etc.) which fade and give way to other trends. And it's typical, as many have noted, to lust for things we imprinted on in our youth. I was a huge comic book fan, wanted to be a comic book artist, and for years collected original comic book art. Some of that art has become valuable (at times ridiculously so), but I can already see that some of it is declining from peak values of a few years ago. Younger comic book fans are only familiar with a small handful of the artists, and a lot of the characters/books the art depicts are out of print or have morphed. Current comic book artists usually work digitally, so there is very little new original art being produced (similar to the transition to CF in bikes). Tastes change, but my pleasure in finding and admiring that art means I got more than my money's worth, whatever those pieces ultimately sell for.
I think when you look at vintage bikes prices of 10 years ago, another factor is the ease of selling these bikes and parts and also trends in the industry. The 90s came and everyone rushed to buy alloy. Then there was the rush to CF. A lot of really nice steel bikes from the that time, and just prior, were put aside. As eBay reached maturity, and Craigslist became ubiquitous, people suddenly had a way to readily convert those old bikes into cash. Word spread that some of these bikes were really wonderful, even valuable, and a lot of people who grew up wishing to they get a Schwinn Paramount or De Rosa had the means to take advantage of this now nationwide and world wide marketplace. This peaked the interest of both sellers, and collectors (it's certainly part of the reason I became a bit of a collector - "window shopping" online). But the number of cool old bikes that were made is in the millions. There's still a huge untapped supply, not of NOS stuff, but of nice old bikes in decent shape. I think 10 years ago or so the interest in the hobby briefly outgrew the readily available supply, especially for high-end vintage examples of certain makers, and prices spiked. And now the boomer generation, who sparked the US bike boom in the 70s, and were a bike part of that price spike, are now aging out of being active riders, or they have their fix of bikes and are super selective. I think the interest overall in bicycling around the world is strong, and mostly growing, but vintage steel bikes are only one small part of that.
I recently built up a trashed Specialized Sirrus from the 80s for my young nephew. He's into working out, and wanted a bike for exercise. I'm afraid to ask him how much he's riding it - it was a gift, and I wanted to give it with no conditions or expectations. Still, I'm hoping it sparks an interest in cycling, and maybe in vintage bikes, but I know it's a long shot. It's just as likely he'll turn to mountain biking, or get into cars, or whatever. He has never watched a racer on a steel bike, nor gone into a bike shop that was full of Peugeots and Motobecanes. Such is life. Personally I find it most sad that year by year knowledge and expertise is being lost. This is the real shame. I never bought any of my bikes (or any of my art collection) with the thought of profiting, or using it for retirement, or anything like that. It's always been about personal enjoyment, and about sharing this love with like-minded folks.
Of course, I'm also a bit of a bottom feeder when it comes to vintage bikes. I spent about the same on my 2012 Cannondale (bought on sale at the end of the season, so not expensive by modern bike standards) than I did on my Tommasini, Eddy Merckx, Panasonic DX-5000, Univega Gran Premio, and Koga Miyata Gentsluxe S combined. If I sold the Cannondale, I'd get maybe half that money back, and in a few more years it'll be even less. If I sold all the vintage bikes, I'd definitely get back more than I spent, despite using all of them. That ultimately the great thing about this hobby. As you said, they're objects of art, and of beautiful design, and they're amazingly functional, and help keep us active and healthy. There's a special pleasure to tinkering with them, cleaning them, swapping parts out, restoring, riding, sharing.
And on that note, I'd love to see photos of some of your bikes, especially the Motobecane and Gitanes.