90% of retro games are not rare. You can find plenty of copies of Contra, Earthbound, Mega Man 5 and Gargoyle's Quest 2.
Yes, there are some legitimately rare titles, however those were (and will likely always be) pricier than the others. Caltron 6 has always been hard to find. And it will always command a higher price than most other things. But it will not keep it's peak value, whenever that may be.
But for the majority of games, there are PLENTY of copies available. If you remove all of the "fad" collectors out there, there have been plenty of copies for most games for anyone who wants one. Now, in the future they will become harder to get, but that's true of anything that has been discontinued. If you want Conta, you can find COntra. Maybe not in your local store, but definitely online. And like everything else, once peak interest passes on something, prices drop. Again, never back to what they were, but I sincerely doubt people will be paying $40 for Alfred Chicken once the allure of collecting has worn. So the price will begin to drop. As to WHEN that will happen, who knows? But it always happens. Retro gaming (not to be confused with modern gaming) is a fad like any other. Nothing stays popular forever.
And the market is definitely not what it was, but it is not devoid of awesome finds. Two weeks ago I found a stack of Dreamcast games which included the MvC titles and Power Stone at a goodwill. It's half location and half luck. If you're in a small town or a huge metropolitan city, chances are less than a suburban area simply due to the amount of competition. Just because retro games will never be what they WERE doesn't mean they will stay what they ARE.