And slowly we spiral into the key issues.
While I didn't have a ton of sympathy for the letter..
As the halfling noted above, WotC could be wrong. The have been wrong about other things in recent years.
3E, and the 3E PHB, were really, really successfull. Former insiders agree on this, industry watchers agree on this. Some data has been presented on it. They coincided with the OGL. The OGL did not hurt them, at least much. Maybe the OGL helped. 3E plus d20 dominated the market like no game has in a long, long time. Again, I think this is "data", don't think its going out on a limb.
Now, I think that the decision to drop the OGL was rationalized by the d20 glut, but was really driven by people in R&D pissed off about other designers "stealing" from them. But I just think this, and base it on little dribs and drabs that have come out.
I do know that 4E has not even come close to dominating the market like 3E. And its main competition is other kinds of D&D. Again, basically data. Not saying which sells more, but the market share situation, at least roughly, is pretty clear.
I think that if WotC had handed Green Ronin and Paizo an OGL type arangment early on, the market would be different today. Maybe there would need to be a mechanism to move certain 3rd party content into DDI...but I think that this would have helped WotC, reduced the market divisions, and they would make more money.
But thats just what I think.
You're not showing causation, though. I do not believe that 4E's sales are affected by the existence or lack of a license, nor by the quality of it.
I believe that any trends are affected to a minuscule degree by the GSL, and that any prime movements in these trends are due to the products WotC is putting out themselves.
Whether we like it or not, the main compaint we see levelled at 4E is not about the GSL, it's that people don't like the game itself. 99% of people who play RPGs have never heard of the GSL; they had not heard of the OGL, either. 3PPs simply aren't a factor in this, in my opinion.
Given the premise that - anecdotes aside - none of us know any of these sales figures, if D&D is selling drastically less product it will be due to the game itself, not 3PPs or the lack of them. It is anecdotally apparent that a lot of people prefer Pathfinder to 4E as a game in itself, and did not like the upgrade to 4E.
So I return to my repeated statement - there is no evidence that altering the GSL would make any noticeable difference in 4E's sales figures. WotC clearly don't believe it would, and all we have to go on is a parallel success of 3E along with the timing of the OGL; but I argue that one did not cause the other. I think that market was ready for 3E, it needed 3E, and lots of 3E PHBs were sold. And, further, that 99% of those who bought a 3 PHB had never heard of the OGL or knew of any 3PPs.
So, in absence of any such proof, there remains little to no motive for WotC to change its stance on this. Especially given their apparent intent to move towards subscription services rather than actual product.
We should never mistake our little semi-informed (yet highly anecdotal) bubble on the web for the whole world.