Howdy. I am bored, so please indulge my long-winded . I respectfully disagree because it depends on the underlying assumptions.
You refer to collections coming up for sale as traditional collector's retire, and you fear that insuffient buyers will remain to gobble up the increased supply, resulting in lower prices. Fair concern, except I would not assume that potential buyers are limited to traditional collectors defined as folks who buy only what they enjoyed reading in their youth. I primarily enjoy GA and SA goodies in my PC, with some BA sprinkled in, despite not having been lucky enough to read many of them in my youth. Why? Because of pride of ownership of books that helped shape current pop culture or are otherwise cool to have in hand. I am sure a bunch of folks also collect books because of the art, stories, characters, whether or not they read the books in their youth, because they remain cool to read/appreciate. For all we know, this group could outnumber traditional collectors some day enough to gobble up any increase in supply, even assuming general population increases aren't enough.
Indeed, we have yet to see a dip in values for the main GA categories or any of the SA superhero titles, despite most collectors today being too young to have enjoyed those books in their youth. One reason could be that Disney/Marvel and DC have maintained the popularity of vintage comic book heroes/villains/stories/art through movies, books, cartoons, etc. And/or folks could be appreciating history more, particulary as it relates to current pop culture, and comic books are a cool way to do that. Because all of these folks expand the concept of traditional collector, supply might not keep up with future demand, rather than the other way around.
I would also not assume that potential buyers are limited to traditional collectors or even collectors in general. Whether or not younger folks apprectiate GA/SA/BA books for artistic or nostaligic purposes as did prior generations, more and more folks of all ages seek out these books for investment. Due to the maturing of the market for vintage comic books, they have been legit liquid assets for two decades now, increasingly so ever since the advent of slabbing, eBay, and other online auction sites allowing for quick/easy sales. This is reminiscent of graded coins/currency or stamps, which continue to do well despite that stuff being old as heck and collectors never having used such issues to buy or mail something!
If vintage comic books will be embraced by collectors and speculators alike going forward, simple economics suggests that they should continue to appreciate for the foreseeable future due to the limited supply. Not every vintage book will be an AC1 or AF15, but over time, many of them should follow suit so long as the market continues to mature and remain transparent. I think this has been true for SA/BA keys for a while already, and recent spikes suggest some CA and modern keys are joining the party. For example, ASM300 and UF4 despite not being rare books. Doesn't this bode well for the scarcer, long-established SA Marvel and DC keys?
So there may be more potential buyers going forward than you assumed, even assuming pure traditional collectors become less common. Or my brain atrophied due to boredom. Anyway, I am first and foremost a collector (and not just comic books), so I am not as concerned with future valuation. But it is fun to stay abreast of market prices nonetheless. The safest bet is to collect the books you like. Here's to hoping everyone is enjoying the hobby, as we near Thanksgiving.