Sorry, MovieBob, but you're flat out wrong about continuity not being a problem. Sure, marketing and accessibility is an issue too, but continuity is a huge, huge problem for the "mainstream" DC and Marvel comics.
Here's the thing about the comparison to other forms of media (movies, books, tv shows, e.t.c.). Those forms do not have have half a century or more of a single long running continuity with vague backstories and interconnected bullshit that constantly comes up.
See, I run into this problem constantly, because while I read comics, I don't always have a lot of free time, so I can go months or years between when I read. And, for example, if I pick up Spiderman after a year or two, I find out he's dead and has been replaced by a black Mexican kid. And no, I can't just go "oh, huh, time to start reading from the beginning of this particular series" I have to figure out exactly how this NEW story fits in with the OLD story, and do freaking hours of research just to find out where I need to start reading from. That's mildly annoying with a well-known character and world I already enjoy for only year or two of continuity. To do it for an unknown character and 50 years of backstory is just impossible.
With a book series, it's not hard to just pick up the first book and read from there. You always know precisely where to start. The series with looser "start" points (like say, Warhammer) are always, as a rule, broken up into much smaller series with, again, clear start points. With most American superhero comics, that's just not the case. Some trades I can pick up and read, like Batman: Year One. But others require a level of familiarity with the material that just may not be there. And since shit is never numbered in a clear fashion, I can't just go "oh i need to read THIS first and then THAT" unless I decide to hunt down and read every single comic from the original #1 printed back in the dawn of time. Which I'm just not going to do.
A similar distinction can be made with TV shows. See, long-running TV shows do this thing where they don't actually require any knowledge of past episodes. If the episode is part of a story arc, they'll spend some time summarizing what happened previously to get you caught up quickly. And even then, they are ALWAYS broken into seasons with set limits and bounds. Even if you are just catching the "999th" of whatever, you can at least start with the first episode and not worry about unexplained stuff from the last season messing up the narrative. And once again, the entry points are clear. You start with the first episode of the show, or the season for continuity based shows, and any episode for non-continuity based shows.
This is not to say that it is impossible to get caught up with a comic series. I did it. But it's an irritating, terrible waste of time and effort that detracts from the value of the entertainment. There are plenty of good, well-written series with decent characters that I'll never read simply because the cruft of continuity is just too cumbersome to get through. I've never read a Green Lantern comic, and unless there's a reboot (not that brightest day crap where I still need to know who the hell all these dead people are), I never will. And that's just sad, both for me because I'll miss out on good comics, and for the publisher because they'll miss out on a sale.
Frankly, I think the comic book industry just needs to sit down and establish a 20 year time limit on stories with the same continuity. You can use the same worlds, franchises, e.t.c., but once you hit that 20 year period it's time to end things and start up with something new. 20 years works fairly well, because it allows characters to "age out" of the status quo. For example, Spiderman at 20 can be basically the same guy as he was at 16 or at 30. Maybe he gets a car, or has to worry about making excuses to his boss instead of his teacher, but the same basic guy. Spiderman at 45 though, is a whole different dude who pretty much has to have a stable family/career with a wife and kids or SOMETHING.
See, the way I see it, at the end of the time span there's a large series ending event that resolves any and all loose threads, leaving the way for the whole comic to end, or for a new series to start. And each new series would start off tabula rasa, as if it was a brand new superhero with an origin story that needed to be told. Maybe you have an "issue 0" that handles the origin story so that long-time readers can just skip it and new readers can get it on the ground floor.
Using the Spiderman example, you could have Spiderman get married at 32 and that would basically end the original spidey arc. All his rogues gallery would be dead, or in jail, and he'd have a stable, long-term career as a physics professor at some upstate community college who occasionally moonlights with the Avengers.
Then, if it seems warranted, you'd pick up the new Spidey arc with him at 40, solidly married with a couple kids, and the comic wouldn't be about "the fast talking webslinger" but about issues more relevant to people in middle age who need to worry about IRAs and paying for their kids' college.
Or, you could "pass on the mantle" to a new Spider hero (spider girl, spider-teen, whoever) with a new costume, limited connection to the past. And, this is the important thing, never, ever, ever again go back to the old story or reference stuff from that story. If fans don't like it, tough noogie, it's dead and needs to stay dead. They can re-read their old comics or read "alternate universe" retreads if they like. But the chronology ends.
It doesn't have to be an exact 20 years, and most stories should end much shorter. But 20 years seems like a good maximum limit to put on things. For any hero who isn't immortal and ageless, 20 years of actual time will change them too much to stay within the same plot and character arcs.
p.s. the above obviously only applies to comics like Superman, Batman, X-men, e.t.c.