Parmandur
Book-Friend
Do you really think so? I'm not so sure. Lots of DMs are worldbuilders first, so lots of them that have at least what they think of as unique settings probably still buy adventures. And I don't think only hardcore, experienced DMs are the only ones using other settings. The core books name drop Eberron and Dark Sun and Planescape. It's not wildly speculative that gamers new to the hobby might be intrigued by such names and go hunting down used books and PDFs.
WotC has provided actual numbers for this in the past: about half of DMs Homebrew, 25% use the FR and 25% will use another setting. Of that first half, about half make all their own stuff, and half will take WotC products and file the serial numbers off. So, their AP strategy is primarily aimed at the quarter of DMs who use the Realms, followed by the quarter who like repurposing books. All other settings combined are a smaller bae, let alone any given setting. But, the conversion is still easy most of the time.