Say we go with a book every two months. Fairly meaty book, it's going to have mechanics. They are not going to start banging out systemless books after all. Class options, new monsters, new magic items, etc, the list goes on.
After three years, now you have eighteen books. That's a very large page count. That's more than the entire 1e line and that took nearly ten years to achieve. Heck, it equals 1e after the second year.
Not every book has to be a big meaty hardback with lots and lots of rules.
You say that 18 books is "more than the entire 1e line". That is not true. The 1e line includes tons of adventures, all of "classic" Dragonlance, and a good chunk of Forgotten Realms material. For example, in 1985 the following books were released (bold signifies a larger-scale release - hardback, adventure compilation, or the like):
Unearthed Arcana, Oriental Adventures, Battlesystem miniature rules, C4: To Find a King, C3: The Lost Island of Castanamir, C5: The Bane of Llywelyn, H1: Bloodstone Pass, UK7: Dark Clouds Gather, T1-4: The Temple of Elemental Evil, I7: Baltron's Beacon, DL6: Dragons of Ice, DL7: Dragons of Light, DL8: Dragons of War, DL9: Dragons of Deceit, DL10: Dragons of Dreams, Battlesystem Fantasy Combat Supplement (Battle of Qualinost scenario), Lankhmar: City of Adventure, and CA1: Swords of the Undercity.
That's 18 products in a single year, five of which I'd consider large to medium-scale (though I'm not familiar with all of these).
No-one (well, almost no-one) is asking for a Complete Warrior-scale hardback every two months. We (or at least I) think something every month or two would be nice, with only one or two of those books being a big fat hardback.