That claim was a big part of why the OGL was originally created. The belief, espoused by Ryan Dancey in particular, that "network externalities" drove the popularity of D&D.
It was discussed a lot on the "Gentleman's Agreement" pre-3.0 OGL discussion email group in 1999, which I was part of.
The idea was that more people knowing D&D rules meant more people played D&D. And, therefore, if you could make those rules even MORE commonly known, the biggest company with the biggest market share (WotC) would be the one that benefits the most from network growth.