Jürgen Hubert said:
That's probably the most likely occurrence. WotC will probably still get the majority of the customers, but a 3.5 OGL game will have a strong following for a very long time.
And I'd be a client for the latter, personally.
I always thought that a big part of the popularity of D&D comes from... its popularity and the shared experience that comes with it. Basically, wherever you go, a group of gamers you'd find there would most likely play D&D, and if you explain RPGs to a newbie, they'll be more likely to know D&D and understand concepts like dungeon crawling, XPs, levels, et cetera. More power for you as a gamer.
3.X through the OGL (and its huge number of variants and refinements for all tastes and styles) became such a phenomenon that I can hardly believe a huge proportion of gamers would just follow the move of 4E and give up on the OGL, particularly since it is not revocable and there still will be some people to publish stuff for it, at least during the years of transition.
What matters then is how 4E is perceived when compared to the huge mass of OGL variants, adaptability, popularity and shared experience out there. If 4E breaks from 3E in radical ways then a schism, at least for the following years after its release, is a sure bet. If it follows 3E closely, most gamers would probably just move on with the game... or would they? If third-party publishers rival Wizards in quality, and the OGL stuff being published is good, varied, for the same price range and satisfies most styles of gaming out there, this could make a lot of people think twice, I think.