Initial sales aside, the only way for D&D to grow its audience is to keep people playing (and talking about) the game -- in public. People walking into game stores need to see people playing D&D. Students need to see signs for D&D clubs in school. Conventions need to have tables filled with D&D players. Kids need to see their parent splaying, and vice versa. And people need to be enjoying themselves enough to talk about it out loud around non D&D players so those folks get interested and want to give it a try. The vast majority of folks come to D&D through a person who already plays. That is only going to get more pronounced as the niche gets smaller and the hobby shrinks. One way to keep people playing is to keep interest up through a healthy release schedule. If the release schedule gets too sparse I think you will see people continue to play but they will do so in their isolated groups.
Organized play is part of it, but not the whole thing. Some people love D&D but just do not care for organized play. Modules provide a common ground and often a point of entry. But those modules have to be *good* in order to serve that purpose, and arguably have to have the capacity to become classics (I don't think you can intentionally create a classic). Assuming the work is high quality in general, a more robust release schedule of adventures improves the chance of a classic breaking out. Look at Pathfinder: they have lots of APs but it seems they really have two classic APs about which you hear a lot -- Rise of the Rune Lords and Kingmaker. Folks that have never played PF are going to hear about those adventures and maybe want to give them a try, even if they have only ever played 2E or 4E before or whatever. There are lots of pre-3E D&D module examples, from ToEE to Dragonlance to Ravenloft, and 3.x has 1 or 2 (Red Hand of Doom gets mentioned a lot), but I can't think of a single 4E module that is thought of as a classic. 5E has the potential with LMoP (but I think when the starter box finishes its print run they need to put that module out in a more traditional format for it to hit true classic status). I don't think Hoard or Rise are of the necessary quality to become classics themselves.
My point, I guess, is that what a more robust release schedule does is keep a conversation going about the game that helps draw new people in, and if you are going to rely on the 3 core as your primary revenue drivers, you absolutely must bring in new players -- especially new DMs because only DMs need to buy all 3 books.