The Chinese have a recent history of buying AF15s. Now that the Chinese government has relaxed its "one-child" policy into a "two-child" policy this creates twice the demand for AF15. If you hold onto your AF15 for the 25 years or so you mentioned this newly mined market in China will be substantial and lucrative. It should be no problem to find a buyer in the Chinese comic-collecting market making AF15 not only an American bullet-proof investment buy a Chinese one as well.
In regards to your original question of where the comic book market will be in 25 years (and I know this is not a popular opinion on these boards) but I see it tanking. Kids today do not collect comics and therefore will feel no nostalgic value to them as you or I (and many here) do. Of course AF15 will still be collectable much as the stamp collecting hobby has died but the Inverted Jenny Air Mail stamp is still highly prized and valued. I don't hold much hope for other current blue chip comics (X-Men 1, Marvel Mystery 1, Archie 1) as most comic collecting money in the future will go towards acquiring the precious few AF15s that will still be around and not forever buried in black hole collections of the Chinese.
I live in a predominately Chinese area of California and many Chinese children dress up as Spiderman for Halloween and their enthusiasm for all things Spiderman carries over to them dressing up as Spiderman for other major holidays, including President's Day. The last President's Day here a Chinese child actually came to my door dressed as a copy of AF15 (In mylar, not in a CGC case) and his mother was dressed behind him as a cantankerous letter from Steve Ditko. The child's father was also there but was not dressed up. He did have a facial mole with a long hair sticking out of it. I believe he works for the post office.
So yeah, AF15s are a sure thing.