Third, "everything is core" has been intrepreted by some people to mean that new crunch is "less optional" than in previous editions. Even if this is an unconscious response I think it tends to enhance the completist urge among consumers taken as a whole.
WotC has probably learned their lesson very well, from the 3.5E era of proliferating splatbooks and the TSR 2E AD&D era of proliferating settings.
At times I wonder how well the 3E/3.5E splatbooks actually sold. I remember seeing several 3.5E Eberron and Forgotten Realms splatbooks in the discount section of a few "big box" bookstores in town. It wasn't just one or two copies, but stacks of 8 or 9 copies of each splatbook for $6 a pop.
From this, I wonder how many splatbooks actually did more than one printing. Of the 3.5E splatbooks I've picked up over the last year from various discount and clearance sales at FLGS, big box bookstores, and online, the only ones in my collection which indidicated they had more than one printing are:
- Complete Arcane
- Complete Adventurer (3rd printing)
- Eberron Campaign Setting (3rd printing)
- Faiths and Pantheons
- Player's Handbook 2 (3rd printing)
- Book of Exalted Deeds
- Monster Manual 3
I used this method for determining printings:
Editions and Printings, How to Tell the Difference: a guide for book collectors
Though this method does not tell you how large the actual print runs were. For example, underprinting something could drive the publisher to do another print run or two, if a title sold out very quickly.
Fourth, and related to the last point, is the business model underlying 4e. Roughly, maximize the number of potential buyers for every book by reducing setting-specific material and emphasizing "generic" material whenever possible. Whether this is a good choice (financially or from the hobbyist's perspective) has been discussed pretty thoroughly in other threads, but it can enhance the "too much too quickly" perspective. In short, even if the total amount of material produced were precisely the same as in previous editions, every consumer that isn't a completist already is being marketed more material than before. From the perspective of the individual consumer that can feel like more material has been produced, and it potentially influences the perspectives of a large percentage of the total D&D audience.
This perception of "everything is core" of 4E generic content causing an enhancment of the "too much too quickly" perspective, could backfire eventually on WotC if the content quality of the splatbooks nosedives in later years to being mediocre or outright crap. If at least the content quality of future splatbooks is on par or even superior to the earlier released 4E books, they can probably prolong the 4E splatbook treadmill somewhat longer until the 4E product line inevitably runs out of steam. All these things compounded together may possibly even shorten the market lifetime of 4E D&D, when it comes to selling paper books in bookstores and FLGS.
The big unknown variable in the 4E product line, is the future viability of DDI. Even if WotC ends up releasing less 4E splatbooks in the later years, the DDI service may possibly still be viable. Subscription fees coming in every month is something they have better control of, than the vagarities of the book business (ie. printing, shipping, dealing with returns, etc ...). If WotC is keeping to their commitment of online exclusive content which may not show up in any paper splatbooks, this may possibly be a another selling point for DDI.
I wouldn't be surprised if DDI ends up becoming the dominant cash cow for 4E D&D. With the D&D brand being licensed for possible future video games and MMORPGs, they might even be able to cross market it with DDI. For example, every D&D licensed video game or MMORPG account can come with a one month free trial subscription to DDI.