mamba
Legend
Given that they release several adventures per year and it probably takes anywhere from 6 to 12 months to run a campaign, you do not really need to buy every one, even when all you do play are official adventures.You figure that the majority of gamers out there are buying the new book every time it comes out to use? The sales certainly don't support that idea. The current player base is measured in the millions, but, I really don't think that WotC is banging out millions of copies of every single book they publish. Hundreds of thousands, maybe, but many millions? That would put them in Harry Potter level book sales.
Add to that that only the DM needs to buy it while there then probably are on average 5 people using it (incl. the DM), which brings the number of sales down further.
So you basically have 10% of the player base buying an adventure, maybe, certainly nowhere close to all.
And that ignores that about half of players / DMs play homebrew and the other half might not limit themselves to WotC offers. So at that point you are really more in the 4-5% range.
Since WotC does not publish sales figures, we won't know how much they sold, but that range is a lot more realistic than your 'there are millions of players' baseline.