Odhanan
Adventurer
Excuse me while I wax philosophic here, but as a player, and a sometimes DM, there's really not much left for me to purchase that would extensively add a huge amount of value to the game. I mean, there are tons of splat books, tons of monster books, plenty of modules, campaign settings, and a host of other great products out there, that already add a lot to my game.
I do not feel that way. For me, the d20 is like heroic fantasy as a literary genre. Could I say everything has been done in heroic fantasy? Certainly not. There is always the next guy who'll come with a new idea, a new tweak, a new concept that will make me wonder "hey, now, wouldn't that be cool in my game?"
There is a natural selection in the d20 market. If you think it's not original/good enough etc, just don't buy it. In the end, the best remain.
The only thing that could end the d20 market would be for WotC to stop the OGL (which is very unlikely) or for some other reason (like rumor, dislike for the business, just the stuff looses its buzz for some reason), people just lose interest in the whole thing. Even in the theory of the publication of a D&D 4th edition without OGL, I am not convinced people would stop producing and buying OGL stuff.