I've been hearing the doom and gloom arguments since the 1980s. Back when I first discovered comic shops in NY in the 1970s, the typical shop devoted 90% of their floor space to back issues and maybe 10% to new issues. By 1990, that floor space allocation had largely reversed, quite a few shops having given up on selling back issues entirely. And I remember all the panic then that the back issue market would be gone in a few years. And yet here we are nearly thirty years later and the back issue market has somehow survived this downturn and a few others as well. In fact, I'm not sure that the back issue market has ever been stronger, at least not since I've been collecting.
For me, the back issue crash will come only when the superhero itself loses popularity. Regardless of the medium that keeps the superhero popular (whether it be comic books, movies, tv shows, video games, holographics, or what ever unforeseen medium emerges in the future), there will always be a sizable enough population of fandom that will gravitate to the original source material because the superhero was birthed in comic books and is synonymous with them. For these people, the required nostalgic element that will drive them to the back issues is the superhero itself, not the comic book. I also think the long established and fairly measurable market for back issues will provide the glue to keep them there for a good long time. Once you discover it, it's just A LOT of fun to play in the back issue market, even if you didn't read a lot of comics as a kid.
Anyway, that's just my two cents... I'm really not worried at all about the comic back issue market evaporating in my lifetime. Then again, I'm nearly 56 now so my horizon may not be as long as others !!