The "collectibles market" may persist, but what people collect may change.
I tend to agree with those who mentioned the importance of nostalgia.
Some millenials are collecting Pokémon cards. I think the original game was pretty neat, but I don't have a yen to collect the cards simply because there's no connection to my childhood. Even though I don't want to collect them myself, I get it. If I had been born 20 years later, I'd probably be collecting Pokémon cards instead of comic books.
If I were collecting for profit, I'd ditch comic books for video games, game cards (not just Pokémon but Magic the Gathering, etc.), sports memoribilia, and tennis shoes. (That last hobby I really don't get.)
However, I wouldn't get much enjoyment out of collecting those things. Collecting would become a job.
Comics will probably fade eventually, but by then I'll probably be too senile to realize it. I'll think they're still valuable. I'll feel very benevolent when I donate my comics to my favorite charity, and they'll be like, "Gee, thanks?" Then the boss will be like, "Hey, Joe, some crazy old man just gave us a pile of trash. Find out how much it'll cost us to rent a dumpster."