So that the game has a 10+ year lifespan, no more than a single book outside the adventures a year, with occasional years of two books. Part of the problem is that folks have gotten used to a very short lifecycle for the game. A book or two per year means...10-20 more books in 10 years. That can be quite a bit. It can be extremely intimidating to hand a new player all of this content when they go to make their first character / DM their first game.
In my perfect world - 2 mega-adventures per year, 1 setting book per year (352 page book), and every three years a monster manual; with the player options and additional monsters attached to the mega adventures and setting books (or free pdfs online).
Setting book would be 32 page adventure, 64 pages of monsters, 32 pages of player options (both of the latter also released free online in pdf form), and 224 pages of setting material.
Monster Manual X would be a compilation of all of the monsters released in adventures / setting books + additional monsters.
I asked before, where is the six for 2015?
2014: PHB, DMG, MM, HotD, RoT, Starter Set: 6.
2015: PotA, OotA, ?, ?, ?, ?
Now, unless you have knowledge that between September and December of 2015, we are getting four new major releases, I fail to see how we get to 6 in 2015.
UNLESS you're using a nonstandard calendar (July 2014 to July 2015) to include all major releases as "year one" OR you are claiming that Fantasy Grounds, Elemental Evil Board game, Dragon+, and SCL releases as "Major" 2015 releases.
Assuming you start your years at launch
Year 1: The Core Three (which aren't repeatable), Starter Set, HotDQ + RoT, PoA
Year 2: Underdark thingy, probably another adventure, ??
My suspicion is that we'll get a setting book of some kind (Eberron? I'd have said Al-Qadim before Rodney left), and that the third "book" is really the summation of Unearthed Arcana columns + the player options tied to the four adventures. I think the part that people are forgetting is that the "player options" book has really been broken apart into the articles + the player supplements tied to each adventure.
Yes... Fingers crossed it will be sooner rather than later.
<evil cackle>. Oh, there will be sweet, sweet love coming. (I'm back to my teasing ways again. At least until Mike gets mad at me. MWAHAHAHA)