Wow, I must've missed the monoculture that sprang up, because I could still buy plenty of games not based on the d20 system... nWoD books, Runequest, Call of Cthulhu, Unisystem, GURPS, Qin the Warring States, PDQ, Warhammer FRPG, Reign, Savage Worlds, and so on.
Perhaps it wasn't so much a monoculture as it was you just overlooked the numerous games out there that weren't OGL based.
No, I am perfectly aware of this.
However, from what I've seen, many of these games were eclipsed in sales by the d20 glut. I think in many cases sales of the truly different games became eclipsed by the people going after the 3e market.
For instance, lets take GURPS. I specifically remember the GURPS product line being a lot healthier than it is today. Before, I could count on anywhere from 4 to 6 new books for GURPS per year. Now, it's hard to even find 1 book released a year, and the size of the books is a lot smaller now.
I don't believe the overall gaming market really expanded all that much with the OGL. Rather, I think it diverted gamer money going to 3rd party publisher to those that allied themselves with D&D. And I think the overabundance of product lead to what caused the d20 glut. And now, with the d20 license pulled and 4e going on ahead of the party, I think some of those companies won't survive. In other words, it appears to me the gaming market decided to standardize on potatoes instead of various vegetables once the person who created it shared the seed, other variants died, then a potato famine hit, and now the provider has changed to a genetically engineered plant that nobody else can use.
Granted, my stats could be wrong or there could be other issues, but I am curious to see if somebody could compare the sales figures in the RPG market from the past decade, and see what happen. But I see very little objective analysis of it, rather many people who defend the OGLs benefits are more often than not gamers who agree with its philosophy and are more into the "spirit of open gaming", or who quote the intentions of its creator, rather than publishers or people interested in making money, or people who have sales figures and other elements to back it up.
I guess this is why I am very skeptical of the OGL's benefits.