O'Neil's second argument is much more compelling:
There's this idea at the back of these complaints that comics spending across the industry is somehow a zero-sum game. As much as some may protest, there is at the heart of the matter a core assumption that if people would just stop wasting money on crap they would spend their money instead on Nextwave / Casanova / Sleeper / Love & Rockets / Kramers Ergot. The consumers that makes up the majority of the audience for mainstream comics are not fans of the medium. They are fans of specific characters and concepts, as well as connoisseurs of a specific type of aesthetic experience that can only be gotten from sewrialized heroic fiction. If these characters and concepts ceased to be published, they would probably not continue to be comics readers.
That's basically the problem I see. If book publishers quit producing romance books, would fans of the genre move on to more literary fiction? I would suspect they would instead read old romance novels. Some might scan them and put them on the internet. It might even lead to a wave of amateur romance novelists who publish exclusively on the internet. So yes, I'm saying that, were Marvel and DC to quit publishing, it would just lead to more fan fiction.
O'Neil makes one final point:
You don't win any friends by berating stupid people...
Oh, I don't know about that.
Anyway, upon further reflection on this topic, I really wonder how many of the people keeping mediocre titles at the top of the sales charts are really disappointed. I'm not about to comb messge boards in an attempt to do some sort of statistical analysis, but I suspect the percentage of people complaining about, I don't know, Wolverine is like 4% of the total readership. And for all we know, those online complainers are downloading the comics rather than buying them. I do think there is some evidence to suggest that truly disatisfied readers are willing to drop titles--look at the sales trends on Uncanny X-Men, for instance, or the more recent Wolverine Origins. And really, if online comics fandom was really the vox populi, Peter David would still be writing Supergirl and Brian Bendis would be selling shoes and driving a '73 Dodge Dart.