Gene is right. The overall Art market is going to stagnate and go down due to demographic and cultural changes. But, just like a typical stock market, there will be certain categories of art, from specific stories, artists, or characters, that will continue to go up. It's likely that MCU related art will get a nostalgia bump in 15-20 years, for example. Doesn't guarantee it will, just that it is more probable than not. But, something like Walking Dead art will crater. Was just at a comicon in Michigan where a few collectors came around to Mike Burkey's booth, and were talking about the Walking Dead. Mike had a few Charlie Adlard WD pages in his stack, and I pulled one out to show these teens/twenty somethings. They were like: "Wait? Is that from the comic? I don't even read the comic. I just like the TV show." Marvel, on the other hand, is doing a ton of cross promotion between the movies and the comics, especially to get the comic sales up, or to take advantage of the movie popularity. A lot of young MCU fans also buy the comics.
There are also a lot of independent comic properties that are in the pipeline to be made into movies or TV shows. If any of these become hits, or resonate in pop culture, the art from them may becomes valuable. (Case in recent point, the Chilling Adventures of Sabrina on Netflix - which actually incorporates art from the comic series into its opening credits scene - may stoke interest in art from that book).
What you will see is that the folks who bought some of this art early, will benefit. Problem is, a lot of this speculation is being built into current art prices. But, I think that's always been the case. Watchmen Art was never cheap. Nor was TDK art. It was always premium priced well above the market even when it was new to the market.
All that said, Mike Burkey participated in a panel discussion at this smaller con in Michigan, and only about 10 people (including myself) showed up for it. And I was hanging out at his art both for quite a while going through his stock and shooting the breeze. Not a lot of collectors came by. A few did (including Michael Golden, which was cool), but it was not a big draw, despite all the incredible pieces Mike had on display. It's really a niche market out here in the sticks. If you are not in NYC, LA, or Chicago (or maybe Dallas), it is not really in people's radar.