I think that the problem is that the big boom is over, its time to face that fact. If publishers think that they are going to continue to sell products at the volume they have in the past they are just fooling themselves. Lets take a good look at the overall picture here.
1. D20 and 3rd edition are released along with the birth of the OGL allowing other companies to print material to support D&D through the D20 system, but other games that use the D20 system. This leads to a massive sales of the core books and fresh and new 3rd party materials from new publishers, the market is new and few products are out there, so people are splurging to build up some good reference books for campaigns and optional rule supplements for new classes etc. Many good 3rd party books came out of this, and others were left in the dust to die.
2. Not just a few short years later D&D and essentially most D20 games and rules get an overhaul, this prompts another boom in core book sales and gives previous 3rd party publishers an opportunity to reintroduce their previous products, perhaps polishing them up and fixing some of the short comings. This leads to several upgraded products and new products to be published, and again in my opinion we saw many good 3rd party books and supplements and some good campaign settings come out of this second boom.
3. The Present. Its two years after the overhaul and 5 years, yes 5 years since the release of 3rd edition. The market has many, many, many 3rd party publishers not to mention the many, many supplements that WoTC itself has published in support of D20 and 3rd edition rules.
Now if you step back and take a look at everything and how long it has been, how many more books do you really think you are going to sell, unless you just have some kind of stellar must have product. In my opinion, not many. As a DM I own many supplements, I have bought many many books in the past 5 years, but I can tell you that I have bought only 5 books in 2005, and at this point have no plans to buy any future books that I see coming out. Why??
1. I have a fairly extensive library at this point of monster resource books, character class supplements, several setting books ( D&D Core, Forgotten Realms, D20 Modern, Stargate, Oriental Adventures, and Spycraft ), and several over all game supplements concerning optional rules, new ways to use skills, etc.
2. With my current library I can GM or play several different game genres, as well as supplement those games with ideas given in the books I own or ideas I come up with myself.
The point is I don't need any more books, there are probably very few books in the future that will have any appeal at all, as they will most likely be niche books that are specific to either a setting or dedicated theme. Like most gamers the books I have amplified by a gamers imagination allows me to use what I have to make new things all the time, and I know I am not unique in this or alone. This is why most publishers are probably seeing a decline in their sales and will probably continue to see a decline as most gamers are building up large libraries that they will soon no longer need to expand upon. Sure there will probably be some good books to hit the shelves in the future, but I see that as a 1-2 book purchase per year at best.
All in all unless there is another big boom in the RPG mainstream, that is unless there is a D&D 4th edition on the horizon I don't see how publishers can hope to see the high number of sales like they have in the past. Also as a side note, with all the D20 supplements and optional rules one could make a good theoretical "4th" edition D&D already, without needing new core books, so in truth for the industious gamer and the thinker, I don't see how even a 4th edition could ignite the mainstream success of the 3rd edition release, at least not for another 5 years or so when 3rd edition gaming would be slowing down with fewer and fewer players and a new younger generation of gamers can once again be tapped to lead the charge.
Just my opinion, but I think its a good view on the current state of the market and the D&D and RPG communities.