The way I have seen it, it depends on the era that defines the 'classics'.
Right now, we look at the mid-20th century and find nearly every firearm made in that era to be a work of art. And it's true, the 1950s -1980s, revolvers were immaculate, rifles had to be made with class or else they wouldn't sell, etc.
But look at past decades. Like 1900-1930 era. Only a handful of guns from that period that we truly consider collectible. There are heaps of guns made during those years that you can find for under $200 bucks.
Same with 1866-1899. There is definitely a catalog of 19th century arms that cost a decent fortune, but those are the tip of the iceberg of the various firearms that existed back then. Most of them weren't anything special, and have turned to a rusty finish all over and parts been misplaced the past century or so.
It's the well made guns of each era that specifically become collectible. Mid 20th century we became spoiled and think that all guns become valuable when old, but that actually isn't so. Right now, I see the M9A3, Colt Cobra, Sig Legion series, and the competition rifles becoming the sought after collectibles in the future. I see something like the Glocks becoming a curiosity as well, only because right now 1st Gen Glocks are collectibles.