I think that the D&D Rules Cyclopedia is the best presentation of the D&D rules ever. I don't see why WotC could not combine all core books in a single volume. They already done it in The Wheel of Time RPG, Call of Cthulhu d20 RPG, and Star Wars d20 RPG, so they know how to do it. Quite frankly, D&D is the only game I have in which the core books are spread in several volumes. As a matter of fact, I intentionally avoid game lines like that, as I think they are more the result of greed than real necessity.
Although I would cut the Barbarian (that's a culture not a class), I think all core classes should remain. However, there is no need to the huge list of spells (most of them are hardly, if ever, used by most players), which is the largest portion of the PHB. Regarding the DMG, after you remove the endless tables of NPCs (just leave a link to Buick's NPC generator) and chop part of the prestige class chapter and the near useless essay about dressing dungeons, there is nothing much after a huge list of magic items, which can be greatly reduced (the extra material can go to a supplement). Also, they can reduce the number of monster to what the Cyclopedia carried, all the rest can go to the Monster Manual series. Finally, they could follow the Cyclopedia and add an abstract system to rule mass combat and schematic information about the core setting (Greyhawk).
In this way, they would have a single volume (Dungeons & Dragons RPG), although they could sell extra supplements such as Monster Manuals, Magic Items Folios, and Spells Compendiums to satisfy those who want more goodies in their games.
