So yes, Gary Groth et al. are absolutely right that comics can be High Art. And if you don't like the superhero genre then no one's forcing you to. It's fine. 

But unless there's comics for kids out there -- at least for the smart 12-year-olds that once seemed to be Marvel's core audience -- then the likelihood that kids will grow up to buy Original Graphic Novels in mass quantities isn't that high. 

I don't read any Marvel titles but DC seems to finally be remembering this. Hence the erasure of so much of the New 52 years and, in particular, the writing style of TEEN TITANS and SUPER-SONS. 

I think I get what you're saying but some serious stories really are best done "with pictures." I can't imagine FROM HELL (Alan Moore and Eddie Campbell) or CAGES (by Dave McKean) done any other way. Or Kyle Baker's works (both comedic and serious). One could go on and on.

Are such comics up there with Joyce or Hemingway or Faulkner or name-your-favorite-important-20th-century-author? I'm not interested in making that argument. Are they up there with most serious literature produced in recent decades? Yes, they are. 

Art Comics are a good thing. We need comics primarily aimed at 12 year olds too. That's all I'm saying.

Comic books are a very wide and varied panorama. Although I've not read any, I know that there are European and Asian comics that are indeed more mature in theme, perhaps even qualifying as "graphic novels" more than "funny books." You used the phrase "sequential narrative"... I like that to describe comics with more mature content or format. But I'll refer to SN ('cause I'm lazy!)

Marketing comic books has long been a challenge. When they were available on the marketplace, that standard fare (super hero, funny animal, western, war, horror, romance, etc.) was designed for kids. They were disposable entertainment... plop down a quarter, get two or three books, read them once or twice, and trade them, give them away, or just throw them out. You and I have both heard stories of "My mother threw away my Action #1!"

SNs would probably have been welcome, but there wasn't much of a publicly acceptable outlet for them. Mr. Will Eisner was a genius, and developed the form... but his most popular work was still "The Spirit", which trod the line between comic book and SN. 

This is from fifty year old memories, but I recall that there was the occasional SN or "arty" comic magazine. But I never looked at those (and at eight years old, I shouldn't have.) The stores that sold these were to a very limited audience. The other outlet I can think of would have been Playboy, which experimented with a lot of different art... but not acceptable to the general public. And from this late vantage point, it's easy to see that Playboy was never in the same class as "Hot Jugs" magazine. (And to be fair, I don't suspect a lot of people bought Playboy for the articles... :)

I guess my first exposure to SN quality material would have been Heavy Metal, and other such magazines. Marvel very slowly got into that arena of periodicals (as opposed to comics), and I don't think that DC every did manage it. And even then, the mainstream comic publishers only used SNs as a curious sideline... something for their older readers (a brand spanking new concept at the time) to try, and to give those readers a reason to keep reading comics... since who would think a twenty-two year old man would buy Green Lantern or the X-Men? (I mention them both because I think Neal Adams started changing opinions with those books, along with Batman.)

And I think that publishers have made a gross mistake by limiting their output instead of expanding it. I feel that there could easily have been a mature comics line as well as a standard line... if it were adequately marked as Mature content, then there would be a market for twelve year olds as well as those twenty-two year olds  (or fifty-seven year olds... ;)

Instead, they focused only on the existing readership, and didn't leave much for the new readers. And so... to keep those readers titillated, they have to use dead superheroines.

*We've had quite the festive discussion about what comics should be called besides "comic books." If you check that topic out, you might be entertained. And no, no consensus has been reached on an alternate name for 'em.

And Green Lantern, Plastic Man, and the Spirit, as noted.

The Spirit was one of the varied "alternate" types of comic books; that is to say, a police/detective film noir type setting, exactly as you said. There were some other types that stretched their genres somewhat (Vigilante and Stuff, the Chinatown Kid, or Red, White, and Blue, or even Dr. Fate or the original Vision.) But they still all tried to cleave to the big seller of the day - super heroes.

I read DARK KNIGHT when I was 13. Maybe I wasn't supposed to. The comics shop owner didn't seem to care. I read WATCHMEN at 15...

DARK KNIGHT was probably a bad idea because, well, it's an "adult" Batman story. On the other hand, something like MIRACLEMAN doesn't bother me because Marvelman was a Captain Marvel rip-off who'd never made it to this side of the pond anyway. And the violence in MIRACLEMAN was in no way fun -- it was supposed to horrify you. In DARK KNIGHT you were supposed to enjoy Batman breaking people's legs and such. 

But it was so popular that I inferred that DC said, "We'll make this the REAL future and make this the REAL Batman" - without thought one of consequences, or drastic changes to the character(s).