
I don't think that any of us really wants tons of new Rule books each year, personally I would much rather have a small number of rule books a medium number of supplements (complete elves, the rouge handbook, "what to do with pesky orcs" guide that sort of stuf) and lots of good advantures modules, I like crafting my own world like the next guy but like 99.9% of the western world I don't have the time to do it so quality modules are it.
I think a lot of people would agree with you.
Unfortunately, not enough people would actually part with their cash. Adventures, by definition, are purchased only by DMs: so you've reduced your market to a fifth right there.
Add in the fact that while people might buy every rulebook, they won't buy every adventure. They'll only buy the ones they have time to play; and each could last from a month to three or more months, depending on the schedule. They're not gong to buy simultaneous adventures, either. So one-fifth of your audience will be buying 3-4 adventures per year (if you're lucky - that assumes nobody homebrews and everybody plays published adventures).
It just doesn't work. That might pay for WotC's cleaning staff, but not Monte Cook's and Mike Mearls' salaries, along with the artists, developers, editors, etc. And I doubt they make massive amounts.
Nah, unfortunately the business survives through the sale of player options. And yeah, that sucks - because that's power creep rather than wonder and adventure. But it's how the market works. They produce what we buy, and we don't buy adventures in large quantities.
The beautiful thing is allowing smaller companies to produce those adventures. WotC can't, but a much smaller company can. And a hundred smaller companies can between them produce a vast support network and fantastic materials for the main game which the WotC would never be able to do. That's what the OGL did, and the GSL failed to do. I hope WotC sees this this time round; I have hopes they will because Cook and Mearls have both been treated very well by the OGL.