It should be designed to have an open-ended number of levels of which only the first 12 or so are expected to ever be played. The higher levels are for villains, overlords, and play groups/DMs who want to play at very high levels and are willing to work to bend the system to make it playable there.
To make the game last, design for a very slow progression through those 12-ish levels.
Lanefan
I think this is a very good idea. We usually end up playing the lower levels a lot more than the higher levels, so instead of having PHB 1-3 covering the same levels, you could have different PHB's covering different levels of the game.Now, that said, I would LIKE to see the initial "basic simple core" set (cuz I wants my red box"Beginners" set) just cover 1-10...with, then, an "Expert" set for 10-20...(or 1-5, 5-10 and 10-20...however they want to break it up) and then the "Epic/Legendary play set" (20+) after that. I know I'd get all three...just to have whether I ever use them or not.