Lizard said:As has been said many times before, by keeping people in the D&D "space", as opposed to moving to non-D20 games. It also keeps people in the hobby itself, based on WOTCs late 90s marketing studies.
Since all but a tiny percentage of players of 'spin off' D20 games DON'T already have a PHB, the loss to WOTC is minimal, and the benefit of keeping gamers "close" is great. (Not to mention, as Mike Mearls has noted, serving to train the next generation of designers. Also also, it keeps game companies from investing heavily in house systems which fragment the market and pull people from D&D. Also also also, any innovations in design can then be used by WOTC, without any R&D costs.)
The only problem is that ths is VERY theoretical. Especially with things like True20 and M&M which are as similar to 3.x as GURPS is to HERO. Yeah, they're point-based but really, they ain't the same game

I think that's WOTC thinking. Sure, those True20 players bought the original PHB but since True20 is as compatbile with 3E as GURPS is with HERO, WOTC is NOT going to be able to sell them splatbook X.
As you pointed out, the d20 goal was to keep everyone playing d20 (and let's be blunt, Dancey meant D&D). So even if D&D didn't produce something you wanted this month, next month if they did, since you were still playing d20 you could easily add splaybook of the month to your game and voila, WOTC gets money.
Everyone wins in that scenario.
I think we basically agree on what the goal of d20 was. I think we disagree on if things like True20 and Conan were actually EXPECTED goals of the OGL.
Settings, adventures and monster books, yes.
Rule books that are basically their own games? eh.....not so sure.