I'd still put that on writing - it's hard, but certainly not impossible to write a story with all ages appeal or even different things for different audiences. I'd argue that a lot of current comics are appealing to neither children or adults, but that children are more impacted by the incredibly long continuity that some comics try and keep held together.Eh I disagree in my view the biggest issue with comics is they've largely abandoned the child market.
Comics used to be all ages but ever since the 90s kids have been steadily pushed out in favor of adult collectors and pretentious hipsters * and as a consequence comics have been forced into specialized hobby shops an endangered species in this modern digital age where you can buy almost anything from Amazon *.
My LCS brought this up, parents regularly come in saying their kid likes superheroes and they're looking for a comic and he can't recommend anything on the shelf because it's all targeted towards older folks.
I think the price issue is very overstated a pack of trading cards cost the same as an issue of Spider-Man and MTG is booming for Hasbro last I heard.
Manga and paperbacks are still selling as well so I don't think it's a "print is obsolete" issue like with newspapers either.
However, you're right that they can't even run a kid's comic for kids anymore - fucking GI Joe of all things is this quasi political nonsense and I can't imagine that there's much subtlety there for adults. I haven't read any of it but just on the premise (The GI Joes are rebels and they're rebelling against the tyrannical government controlled by COBRA screams "writer's opinion of the 2016 election" to me) it sounds awful for all ages.
I don't think price is an issue and I don't think "print is dead", but I do think a huge availability of cheaper and easier to get entertainment isn't helping the comic book industry at all. I think that you're right that if a child walks into a comic shop with $4 the last thing he would likely buy would be a comic and that's a pretty large problem. I think comic shops already have a really hard time getting people into their stores but not having much to recommend once someone does wander it and most comic stores near me that survived became something "other" than comic book stores. Most of them have half the store dedicated to comic-related merch, some of them are comic/WOTC stores (D&D and MTG), some of them are comic/video game stores, one of them became a fucking comic shop/lawyer's office/public notary - it's wild what some of them have to do to survive.
Manga doing well only tells me that if you put a decent product out there, people will buy it. I personally think that Manga is handled much better than comics because they don't do nearly as much canonical nonsense - they're all (comparability) straight forward stories that go from #1 all the way to #current issue. There aren't many wacky reboots, no variant cover nonsense, not many canonical cross overs or full universe realignments, etc. The art and writing is usually much more consistent and you much less frequently get writer change storylines that consist of "forget all that shit from the last 12 issues, this is the new shit now" type drastic changes.