Foo Foo,
I agree 100% about teaching the next generation to cherish the past, and taking that into consideration when pricing your treasures for sale. Investing in beauty that you believe in, knowing it will give a return some day....I love the way you put that!
As for the 20 year cycle, that has not been my experience in dealing with vintage and antique clothing. With other antiques, maybe so, but not so much fashion which is influenced greatly by what is going on at the moment in the culture. I have sold vintage for over 40 years and I find no distinct number of years for a cycle, although certain items do rise and fall in popularity every so many years as trends get replaced and new young buyers come into the market. But 20 years is too long of cycle, especially since 40 years ago there was almost no vintage fashion "business" to speak of, just a few shops and random dealers who had foresight. And the hippies who bought it, and wore it out to rags.
Antique purses seemed to be THE big new thing to collect in the early 1990s and prices just got higher and higher. I was always finding them at rummage sales, yard sales and even shops and for low prices too. Books and price guides came out with sky high values, many collectors sold off their collections and made a killing, but 10 years later prices fell like they almost always do. But you know all this!
As you say, a well made thing of beauty is a joy forever. I am sitting on hats that I will sell someday, prices are not a factor, I just love to collect them. In time, hats, purses and other accessories will rise again in value.
I just hope it won't be another 17 years!!