Looking at that page, I think it is evidence there are few. Two of those are WotC's own SRDs. Another is FUDGE, and two more are variants of FUDGE (and as I recall, didn't FUDGE preexist WotC's OGL?). Two more on that page are simple compilations of d20-compatible Open Content spells and monsters.
Leaving three on that list with direct d20 SRD derivation (True20, Anime20, and Perfect20). Has anyone ever published for Perfect20?I am honestly curious. Guardians of Order went under, so is anyone else using Anime20 now? Leaving True20 still actively supported not only by its creator but by other 3rd parties as well.
There may be more other complete SRDs, but that list seems to reinforce "few" other WotC-SRD-inspired SRDs. We all may have varying thresholds for when "few" becomes "many" though.
From that perspective, I agree. Personally, I think the fact that few have provided such SRD's shows a lack of confidence in the use of SRD for their product to this day and thus shows that WOTC may actually prefer not to give out information so readily as they did with the original d20 SRD did. That would be circumstantial support to why the OGL is not a flawless document, and to bring it back to point, it shows a motive on WOTC's part to correct the ill.
It seems to me that OGL has been fairly unique to d20 outside of Fudge (which is a d6 system run by a guy out of his garage, and thereby knowing little about what's good for business). I think the other companies relize that the OGL isn't as influential as many seem to think. A 3pp sees an OGL for the largest game on the market and sees a free ticket to ride. Now here's where the OGL works for D&D, but wouldn't work for other systems run by smaller companies. The OGL proliferates these 3pp's, diminishing the share of companies with independent systems and thus eventually putting them out of business.
Now, as a corporate entity, WOTC should have made the OGL so that it wasn't "perpetual" and shouldn't have made it so that you can still use an old OGL, meaning that changes to the OGL mean nothing. What WOTC could have done there is expand the d20 market, while drawing competitors in, then when d20 is has forced non-d20 systems out of the market, then cut competitors off from the system, collapsing them and D&D stands upon a mound of corporate corpses. That's how the GSL could be manipulated. But considering that the d20 OGL is perpetual, it makes it impossible for WOTC to execute such a plan by any license at all, as d20 will forever hold the lion's share of the market until OGL's for other non-d20 systems proliferate.
Now how does WOTC provide a product that takes the lion's share of the lion's share? If they provide a product that is just like everyone else's, then there will never be a lion's share of the lion's share. What they have to do is provide a product superior to d20 classic and is superior to all other systems, so they are back to square one. The water levels may have raised temporarily, but what happens when you replace one liquid for another? You're still in the same boat. So now they have to produce a superior product to what was already considered a superior product. But now they aren't the only ones with the original superior product. Now everyone has it and they must produce a product even more superior and indeed superior to the improvements that others have been allowed, even encouraged, to make.
The question now becomes How to do that? Answer: Create a system that is not only superior, but different, then give your core system a license that invites companies to use the system, but which requires their customers to use your core product to get the fullest benefit.
The OGL market is stagnant (not in the sense that no one's making money. The problem is that EVERYONE has the same opportunities at making the money, no different than before the OGL. The OGL could only grow things so far. To quote Syndrome: "When everybody's special, then nobody will be"). There's only so much money that can be made there. But if companies help WOTC make money, then they can make MORE money, and WOTC gets even richer. If they had done that from the start instead of the OGL, WOTC would be a giant far surpassing what it is now, and the OGL would be no cure. In other words, the GSL will be what puts WOTC back on top of the heap, because there will now be a heap. No one is ever king of an ocean, but there's always a king of the hill.
Trust me. WOTC
WANTS 3pp to want to make products through the GSL. They also want it to be even more attractive than the OGL. Because with the GSL, the more money 3pp's make from the GSL, the more money WOTC can make, because they are selling the core rulebooks. It becomes money forever.