There's a kind of formal fallacy called
fallacy of the single cause Just because they have inner problems it does not mean there aren't external problems as well, that can be, and should be, dealed with regardless of the inner auditory.
Only a fallacy of there is evidence of external causes which I am not convinced there is. Nor have I said there is a single internal cause, since I've cited their gutting of their institutional memory, shunting talent off to other companies in the industry, moving away from the OGL, creating a double-sided PR nightmare with the GSL and the denegrating of 3.XE as they moved to 4.XE, and I'm sure I could come up with a few others, and some other folks would likely come up with more, but that's besides the point, IMO. I've refuted the belief that the external situations are causal in their circumstances in any significant way, and though I understand the arguments to the contrary I don't feel the evidence supports those arguments.
There's evidence that they built a new 3.5 edition after 3.0. They did so for a reason. If they were earning as much money as they wanted with 3.0, they wouldn't had made 3.5, or 4e.
That's evidence of an internal decision but note that they continued using the OGL with 3.5, initially.
I wasn't comparing them either. Warcraft rpg and world of warcraft rpg are, both, d20 games. So were swords and sorceries, mutant and masterminds, conan d20 and so on.
I addressed this in a previous post reply to Nellisir.
The fun part is OGL gave tabletop RPG business the technology to build solid, good hammers. It's hard for some chinese car company to build a car that can compete with Ford. But if Ford makes his enginery Open Content, it's trivial to build one and just change the color. I probably wouldn't had bought some random "Conan" game or "Elric" game, because a lot of those have crappy mechanics. But giving those the "d20" logo made me sure that they have a solid base to start with. Much less room for mistakes. OGL is like allowing local soda companies to put a label that says "made with CocaCola formula"
I don't believe the OGL gave folks outside of WotC the ability to create any better hammers than what was done prior to the OGL in the form of things like GURPS or the White Wolf games, none of which I see as having been a threat to D&D dominance and, as I said, I'm also not convinced it is a zero sum market.
The fact that the only ones who have real data about it (WotC) have a different stance is a hint, however.
The only non-WotC sources we have suggest that 3.XE (all OGL) lasted 8+ years and that 4.XE (non-OGL) has lasted about half that time, and that in recent years the first real competition D&D seems to have is from an OGL, non-WotC D&D-like game. Though, again, I'm not convinced that the former WotC employees of Paizo couldn't have created that competiion without the OGL. I'm more convinced that the lack of customer support for 4.XE opened the door for Paizo (OGL or no) and also prompted WotC moving on now to create a 5E. WotC moving on twice as quickly from 4.XE to 5E, as opposed to 3.XE to 4.XE, does seem to offer us a hint.