In this regard the OGL was a complete success. But consider other d20 products like Malhovic's Arcana Unearthed (and it's successor Arcana Evolved) or Green Ronin's Mutants and Masterminds (and it's True20 evolution). These are d20 products for sure but consider this:
I ran a Mutants and Masterminds game for 10 months. I had four player's buy the Mutants and Masterminds rulebook. I had two player's pick up Ultimate Power supplement. I myself purchased the rulebook, the pocket version of the rulebook, the gamemaster's screen, Ultimate Power, the M&M Gamemaster's guide, and several adventures.
No one bought a PHB, a DMG, or MM. No Races of... or Complete... were purchased. WoTC did not make a dime out of my game.
And yet I had character sheets that had Str, Dex, Con, Int, Wis, Cha. I had feats, I had skill ranks, I have Fort, Ref, and Will. There were DC's and an 18 Strenght meant a +4. And, of course, we rolled a d20. Sure there were changes to the game (really GOOD changes btw), but this was clearly a game based off of dungeons and dragons d20 mechanic. And yet WoTC derived (and would derive) zero cash flow from it. I had several player's who never even played D&D (and still haven't).
Did M&M or AU ruin WoTC financially? Of course not. But I can't imagine the profit margins in the RPG world are so vast that you can afford to give ANYTHING away.