That is what they said at one point, and they were probably right at that juncture. In 2e and definitely portions of 3e there was a lot of glut of campaign settings and modules. I think some things have changed though. A surprising number of people, though I'm not one of them, are playing in game stores and such and going through the adventurer's league and such. This has some bit of a "shared experience" that seems to be appealing to people. I can talk to someone about "Storm King's Thunder" and they probably know what I'm talking about and maybe are playing/running it. It's at least more ubiquitous in my experience right now (anecdotal as it is) than say Red Hand of Doom was in 3e days.
If you go way back I recall stories that Gygax at first didn't really expect modules to sell, he thought everyone would want to run their own worlds, create their own fantasy experiences, etc. And he's not wrong, but there's also something to be said for ease of use and that sort of shared experience thing.
Popular adventures started early, I believe Keep on the Borderlands sold into the millions of copies. Temple of Elemental Evil was also a popular one. I guess you can make your data tell you whatever you want to see.