I'm not Ryan, but I'll toss in my worthless tuppence on this: I'd say that just as much as the new system traded in on the in-place network of D&D players, it itself contributed to the return of sales success to pre-1990 levels because of the so-called
"Skaff Effect" which was accelerated by the creation of the OGL and the d20 STL. D&D definitely was languishing between 1990 and 1997, and if (despite edition) all other products ultimately drove D&D sales because it was still #1 despite languishing, then the OGL make the masses return to it over and over again, even quicker. If I'm quoting Charles Ryan correctly, last year was the best D&D year of sales ON RECORD (as in, ever.) Whether it's true is up to the sales figures to be believed, because TSR didn't keep as good a track of sales in the early years, but even if you don't take that as true, it restored the sales of the game as a whole to levels unseen in 15 or 20 years, easily.
Therefore, for as much as d20 would have succeeded or not, it and the OGL had as much effect on D&D as D&D had on it. Without it, it may not have gotten its foot in the door, but when it did, it added it's own draw to the picture.