A few factors contributing to continuing C&V value.
1. People who grew up in the 60s-80s who liked to cycle, now have the money to buy their grails.
2. These bikes were thrown out in the late 1990s and early 2000s as aluminum/carbon came on the market, making high-end steel seem obsolete. Lots of supply is now gone because of this.
3. Young people with little money, and wanting a single speed/fixie - buy these old frames because they are more easily convertible (and cheaper) than modern bikes. Their friends, seeing that they have a "cool green Bianchi!", end up also wanting a cool character bike, and therefore end up desiring a nice C&V bike.

The above reasons mean that mid/old-age folks who grew up on these bikes desire C&V, and many "young folks" in their teens and early 20s, also desire C&V (heck, THEIR grail may also be a 1980s celeste Bianchi that they never even grew up with, but their friend has one and it's the coolest bike ever!), meaning there won't be a demographic drop out in 20 years for buyers of these bikes (compare to collectors of sewing machines and typewriters, who are all in their 80s or dead).

I've followed lots of other collectibles (e.g. comics) and in almost every circumstance, everyone in the hobby is graying and wondering who will buy their comics in 20 years? That concern it seems will not be an issue at all with quality vintage steel.