I believe that the Direct Market Boom was more harmful in the long run than it was helpful. Sure, it built up and peaked in the early 90s, but there was nowhere to go after that. And no new readers seeing an interesting Spider-Man or Batman book on the spinner rack at 7-11 to keep the fire stoked.
Technically, the industry has been in a constant state of decline since WWII. The overall sales of 1950 were half of what they were in 1940, and that percentage remains constantly falling every ten to fifteen years. I remember when people were dropping their jaws when X-Force #1 and X-Men #1 had million-copy print runs in the early nineties. That was the median average print run of most moderately successful Golden Age DC titles circa 1940. It took 50 years to get a handful of issues that matched the average print run of Golden Age books, and 30 years later a million copy print run sounds like an impossibility.
The world is constantly evolving. People in the 1930s didn't have television. Comics grabbed their attention, and most anyone could spare a dime for 64 pages of color comics, even during the Depression. And everyone read comics: adults, kids, male, female, black, white, brown, etc. They were ostensibly written for kids, but they were enjoyable on an adult level, like the best pulp heroes and serials.
There was cross-media promotion from Day One: you could watch Superman battle the Molemen on the silver screen in movie form for adults or Saturday Matinees for the kids. There was an animated Max Fleischer Superman short that played before every big movie. You could hear Superman on the radio and read his daily newspaper strip. All of them came with the tagline reminding you to "Read Superman's monthly magazines published by National Periodicals!"
Adjusted for inflation, we should be getting 64 pages of full color comics today for $2.00. Instead they're double that price for half the page count. There's problem numero uno, and it's a huge problem. All the other problems are ancillary. Availability, story, art, lack of newsstands, kids not reading anymore, etc; these are all real contributing factors to overall decline in comics. But nobody feels that they're receiving any value for their money anymore. It pushes comics directly into the realm of adult collectibles.
When adult collectors move on for good, who will be left to buy comics? It's the same question I ask about the action figure industry. Comics, toys, trading cards and the like are all fueled by an ever aging adult market who collect out of nostalgia and have disposable income. Kids today aren't growing up with comics and action figures, so 20-30 years from now they'll have no nostalgic need to collect them. I see action figures and comics being viewed in 2050 the way that kids of the 80's viewed slot car racing and model railroad collecting: old, weird and boring.