So given that, is it really so difficult to think that a terrific, quality 3PP product for D&D would encourage someone to play D&D, (and buy books, subscribe to DDi, etc.)?
Several of the posts I've read recently have made me start to change my mind on this.
I am absolutely certain that it worked great for 3E. The OGL blossomed vast variations that were available. People found those new options and expanded their games into more areas that appealed to more people.
But that is a feedback system with a fairly low yield. If Joe and I each buy PHs and then Joe buys a cool new 3PP product to take the game in a new direction, I'm not going to buy a new PH because of it. It is only when a NEW person buys that things grow. Though, admittedly, that is only one aspect. If Joe's new game keep me playing 3E instead of moving on to Warhammer as my primary game, then two months later I buy the latest splat book, WotC has benefited. But still, the point is that the yield is low.
Clearly, for 3E it worked. But would it work for 4E? My reflex answer is: of course, why would it be any different? But that may be wrong.
For one thing, with the split market gives a smaller foundation. Maybe the feedback system just doesn't have a critical mass to move forward. Of course if that alone answered the question, then it wouldn't be working for Pathfinder either, since Pathfinder is somewhere in the ballpark of the same market.
But also, we keep hearing of 3PPs doing well in PF and not in 4E. (The exception of ENWorld clearly noted). Maybe this yield = X% and X is not the same for 3E as it is for 4E. Fiery Dragon was a solid name in the 3PP field during 3E. They produced a 4E product and recently said that it did not sell well at all. Why not? And they seem to be extremely typical. (Again, ENWorld seems to be the classic "exception".)
Perhaps 4E fans (as a collective market, I'm certain major exceptions exist) are not nearly as inclined to buy 4E stuff. I know I've been in numerous discussions in which it was more than obvious to me that the typical 4E fan sits down at the table with very different expectations than myself. Maybe that plays into it.
So a smaller base may be a problem. But if you are using a feedback system and you turn the gain way way down. It isn't going to perform.
Granted, this analysis is not going to be anything WotC presumed. If anything they assumed the opposite: A) everyone and even more will play our new game and b) 3PPs might take too much of our market share. So I'm certain this thinking has nothing to do with how we got to the GSL. But it seems to model our current position.