You have to take into account more than just the sales though. You also have to account for the costs of production. It seems to me that churning out book after book of splatbooks, one after the another, is a lot more expensive than producing a couple of thematic or setting books a year.
Furthermore, there's also the possibility that too many books - as evidenced by 2e, 3e, and 4e - can have a burnout effect, where gamers gradually became less and less invested in an edition because of a preponderance of new rules and new material. I honestly think 4e's "must-buy" attitude towards not just the original core three books, but their second and third iterations, as well as the "Essentials" package, probably hurt it more than a little. It certainly didn't seem to help the brand.
Based on the current approach of a slow reveal, it seems WotC may agree.
Well Pathfinder kind of prooves your theory wrong about burnout because they steadily churn out books and the people keep buying. You don't have to buy these extra books. I have never understood the compulsion to buy all the books and then complain there are too many. There is nothing stopping you from setting book limits for your games.