Now I tend to be bothered by blanket claims that the comic market is shrinking. This article
http://www.comicbookresources.com/?page=article&id=32716 brought to my attention here http://www.legionworld.net/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=8;t=002356 points out that, yes, individual books sell less copies on average, but there are more titles. (I'm not sure in which way you meant your statement, so I may be agreeing with you here.) Obviously, more titles have to cost the publishers more to make, so this shift isn't good for the industry. I can't reasonably ask everyone to buy the same few books, though. Lack of choice isn't a benefit to the reader. But the only way a market can be expected to support an expanding number of products is if the total market is itself expanding, and that's not the case.
Anyway, I guess comic-cons are now like comic movies: attracting more attention than the comics, that is. I mean, cons are more public and visible, and they're also considered *more* nerdy and weird, which is why they get attention. But if they reach the stage of being regarded as more "mainstream"* than comics themselves...
I guess it's part of the same general phenomenon I have to admit I participate in. Increasingly, we like talking about fiction and doing ancillary things related to it than reading the original fiction. I suppose I could compare this to what I've observed with movie releases. Since the 90s, publicity material and trailers seem to be coming out ever further in advance, while theatrical runs grow shorter. The actual premiere day feels like a non-event.
*I really don't like the term "mainstream" either. As pop culture increasingly orients itself towards formerly "nerd" things, that distinction doesn't seem to matter. Last I checked (which was some years ago...) the top TV shows were taking a significantly smaller percentage of the audience than decades earlier. Simply, there are more channels and more shows. The difference isn't as great as with comics, but it is significant. What I'm trying to say is, the market is a little more friendly to TV series (or indeed fiction in other media) with somewhat narrower audiences - helped no doubt by more careful marketing. At the same time, the Internet has greatly aided the spread of fan culture. I've heard this described as "Everyone's an expert on something", often with additions like "or (likes to) think they are." Again regarding TV, another significant shift is toward increased continuity and serialization. Producers recognize that more of their viewers care; there are fewer casual viewers.
Anyway, I guess much of what I'm saying is vague - I was going to say platitudes, but that's not the right word at all... I suppose, to move back toward the original point (slightly), it seems that comic fan culture is becoming increasingly accepted (again, another vague statement that's been said for a long time). While that may sound good, though it's not if you value feeling part of an exclusive club**. The puzzling (at first glance) question then becomes "Why, with 'nerd' no longer a bad thing and with Hollywood on its superhero kick, are the comics themselves not benefiting?"
And yes, I have read about this, because it's one of the most talked-about subjects in comic fandom. It seems that comic publishers are just *really bad* at marketing, which explains why all the ancillary stuff thrives while the source material struggles.
Maybe I'll post some more specific stuff later, but you did start with a rather broad topic.
**That was what the pre-Internet fan experience was (supposedly) like. Fans of something knowing things most people didn't. Incidentally, this exclusivity, or lack thereof, was brought up in Legion fan circles before in a metafictional way. Threeboot vs. original: the original Legion was an exclusive club, the threeboot said "We're all Legionnaires." While simultaneously a fourth-wall-leaping gesture to the real-life fans accepting them into the club, it turned some off because said club wasn't exclusive enough anymore that that "membership" didn't matter. Can't please everyone (or maybe anyone).