Wasn't one of the justifications for the 4e approach to the Realms exactly that - that the sheer volume of accumulated lore had become a barrier to entry for too many people?
That was the thought.
People didn't like all the available lore, so they radically changed the world. People didn't like all the high level NPCs, so they advanced the timeline and killed most. People didn't like the crazy number of gods, so they removed most of the side pantheons. Etc.
But removing the lore didn't bring waves of people into the Realms. The people who complained didn't stop. And all it did was alienate the fans of the old Realms.
People dismissing the Realms for having too much lore is really an excuse. They don't want ti get into the Realms so they say that. The problem is more players knowing more setting lore than the DM and contradicting them, which is a problem that can't be solved by WotC.
All books sell for a very short time unless you have a sudden surge from word of mouth. That's the entertainment market in general. The initial sales has to meet the target or the project likely won't be deemed a success. Class books are popular with a much larger segment of the audience. It would be odd for them to make a the usual amount covering the different classes given the money makers they usually are in a tabletop RPG game cycle.
Some books continue to sell well after their release. Core Rulebooks and the first big accessory. For Pathfinder, the
Advanced Class Guide still pops up on Amazon and Paizo's top charts every so often. That's what you want: continued and sustained small sales.
Splatbooks appeal to a minority of the audience. The majority of people who bought a PHB will not buy a splatbook. Arguably majority of gaming groups might (eventually) buy
a splatbook, but which splatbook varies depending on their needs. The more options there are, the more you're spacing out sales and effectively competing with yourself. And even the people who do buy multiple splatbooks will eventually stop buying. There's a finite number of accessories any one gamer will buy: each time you release a new splatbook, odds are it will sell fewer copies than the preceding book. After enough releases, you're unable to sell enough copies.