But I still think that the maxim "adventures don't sell" holds, if compared to sales of rules.
A few things to consider:
(1) Adventures are likely to be less successful in games which don't feature a unified, default scenario hook. Games like D&D and Shadowrun have this, but a lot of others don't. (Transhuman Space, for example, goes all the way to other extreme and people routinely report having difficulty figuring out anything to do with it at all.)
Without the unified, default scenario hook, the odds that any given adventure will actually be usable in a particular ongoing campaign is incredibly small.
(2) Building off of that point, Adventure Paths and similar "campaigns-in-a-box" may have a larger potential audience. (You don't need to figure out if a particular adventure will hook into your campaign; the adventure product is the campaign.)
OTOH, this requires the existence of a significant audience that wants a campaign-in-a-box. If there isn't (or you can't build one), then the audience for your product is essentially zero (it can't be used as part of anybody's campaign).
(3) It's probably also true if you're looking at product-vs-product sales. But if you're interested in building a sustainable, long-term business plan then adventure products may out-perform rulebooks: Since the splatbook product line became popular in the early '90s, every single company that has adopted it has burned out (usually in the range of 10-15 products if they're successful, maybe a few more if they're really successful) and been forced to reboot the line. AD&D2, PEG, White Wolf, AEG, WotC, and so forth. It's a long list. (One could go even farther back and say that the original
Traveller suffered the same fate.)
And, notably, most of these reboots are not successful. With a couple of notable exceptions (which featured
"clear, deep, and widespread dissatisfaction" in the customer base), these companies always end up splitting their existing customer base and playing a game of diminishing returns.
Unlike rule supplements, however, adventure products get used up: Whether you're actually playing them, looting them for usable material, or just reading them for pleasure there's always going to be a need for another adventure.
(4) If you're actually developing your rules properly, they cost a lot more time and resources to develop than adventure products. (You can, of course, develop your supplementary rules on the cheap... But if you do that, you're just creating other sustainability issues in your product line.)
So, to sum up in slightly different terms, I think Paizo's secret to success is two-fold:
(A) Produce products with a high quality that people can rely on.
(B) Don't kill the golden goose. (Both by avoiding the splatbook burnout cycle, but also by pacing their release cycle in order to keep their loyal fans hooked without overwhelming them.)
The Pathfinder core rulebook may sell 5x more copies than any individual AP. But they publish 12 AP SKUs per year. You do the math.