Back in the 60's and early 70's it was well known that if you bought good name brand wood and blued steel, you would be set in retirement. There were Browning, Weatherby and other high dollar rifles and also the classic Colts and S&W to choose from. Then there were the recent desirables like pre-64 Winchesters that shot up in value. Most of us could only afford a few of the standard offerings or mil-surp or used... I remember looking in a Gun Digest at a Colt AR-15 and the high price at the time, thinking " what an ugly gun - who would pay so much for that".
I collected S&W for a while and they did appreciate but when compared with inflation, not as much as it may seem. Most of the other rifles from the wood and steel era have not held there own like promised.
Like stamps and coins there weren't the numbers of new collectors coming into the market to buy up everything the old timers had collected at the new prices. Also technology changed with the plastic guns. Like the article mentions, the rare and famous will always command a high price and the pristine condition will probably hold value but the regular guns most of us shoot are utilitarian tools until their brief day in the spotlight.
Today's market is fickle, whats hot changes weekly and many people seem to think their used gun is worth new prices. Sure, I've made some money reselling a few guns here and there when the market is hot but not often enough to make it a reliable supplement to SS, like the original plan.
ETA, it took me 5 years to sell a MB- SL500 but the right guy finally came along with the cash. Sometimes you just have to wait. "You can never ask too much... Only too soon"