Spider-Man came after Fantastic Four and for most of the '60s FF was the top and Spider-Man had to develop word-of-mouth and identity under its shadow and slowly usurped the FF. The X-Men were marginal until Wein/Cockrum/Claremont/Byrne in the mid-to-late 1970s where they took the least successful of '60s Kirby-Lee titles to making it the hottest comics of the market. Spider-Man and X-Men earned their success on word-of-mouth, by developing its own niche, through the commitment of its writers and artists. Whereas The Avengers' biggest period of success is "
astro-turfed" for want of a better word. When Spider-Man developed it was a little known writer-artist called Steve Ditko who made it work and gave it its engine. When X-Men became big it was unknown figures like Claremont/Cockrum/Byrne. Whereas The Avengers got the red carpet treatment when Bendis came in. Marvel's biggest new writer, who got access to Spider-Man and X-Men, and basically all of Marvel Editorial and Promotion bent itself to make Avengers happen.
So that doesn't strike me as democratisation, that strikes me as Editorial putting its thumbs on the scales. Now of course, to some extent all editors and publishers do that but it's hard to ignore the number of second chances and special pleading and considerations and gimmes that The Avengers got. It's also hard to ignore that The Avengers got to change and alter the status-quo of Spider-Man and X-Men comics like Spider-Man's Identity being outed (which JMS, the writer of the time said should have happened in the pages of Amazing Spider-Man as it is by the norms) and the Mutant Population being decimated because of a New Avengers event. Remember historically, The Avengers titles focused on characters who didn't have titles because they couldn't upset the status quo of the Big Three (Thor/Tony/Cap) but somehow that same courtesy was denied to Spider-Man and X-Men when they joined in, and the Avengers didn't pay things forward.