The mythic quality of superheroes comics in terms of the characters continually renewing themselves to fit into the present, never really aging and having no meaningful long-term memory of their past adventures is something Umberto Eco touched on in his essay on Superman in the early 1970s. Of course at that time superhero comics were almost entirely done-in-one short stories and the shared universe wasn't as much of a factor as it later came to be.
I don't think the monthly format is the reason why superhero comics from the Big Two take the form of open-ended serialized drama in a tight-knit shared universe, nor do I think switching to original graphic novels would put an end to that form of storytelling. After all, there are a lot of non-superhero monthlies that work towards definitive close-ended conclusions and writers working on superhero comics typically "write to the trade", so a switch to original graphic novels for the Big Two superhero universes would probably just lead to a series of connected graphic novels with the same kind of serialized drama and tight-knit continuity that we currently see in superhero monthlies. It's not the format, but the readership that dictates content, and continuity-heavy serialized drama is what's favored by the core readership of superhero comics these days.
When the speculator market collapsed and comic book sales steeply declined in the mid-1990s the industry shifted to a niche hobbyist market with a small but largely affluent readership. While sales for individual titles dropped, the number of titles the Big Two published rose, as did the price point comics were sold at. So, to answer the OP's question, the comic industry stays afloat by selling a large number of titles at a high price point to a relatively small readership. That's a strategy that has put the Big Two into profit (even when one looks just at their publishing revenue) even as other sectors of print media increasingly go under water.
Most of the suggestions thrown around in online discussions (like advocating a switch to manga-style publications) are unrealistic because of market conditions. The comics market as currently constituted has provided a bridge of relative stability while the elements fall into place for what could and should be the next major evolution of the industry, which is digital comics.
For some years the Big Two have been criticized for moving too slowly to get into digital comics, but really the platforms available - PCs, laptops, e-readers, etc - all had distinct disadvantages that would have impeded a major push for digital comics. That, however, is going to change over the coming years as the iPad and its competitors go through multiple generations and dropping price points and eventually emerge as ubiqituous computing devices, devices which are perfect to read digital comics in color and displaying a full page at a time, maintaining the multiple-panel page designs of print comics (and perhaps with different reading options, like switching between a motion comic display and a traditional static display).
Now, the industry will need to be careful in how it manages a transition to a focus on digital as one wouldn't want to jeopardize the existing direct market before a viable replacement business model was in place, but digital comics certainly offer the potential for comics to once again be a mass medium with a relatively low price point (although the lower price point will likely only come once there are sufficient digital sales so as to be a replacement for lost print revenues). If superhero comics were to achieve a mass medium readership again through going digital you'd probably see a move back to the characters and their universe being pared down and simplified with more of an emphasis on self-contained storylines rather than ongoing serialized drama.
