It's also impossible to know how 3E would have fared without the OGL. It is entirely possible that it would have been a flash in the pan and then just fizzled and there never even would have been a 4E.
I'm a 3PP skeptic in the sense that I actually don't think 3PP have had much impact on the success of D&D with one huge exception: Pathfinder 1.0. I buy 3PP but I think it's a niche market and D&D's recent success is largely built on:
1. Demographics, with the original mass D&D cohort of the 80s reaching an age where they had the time and disposable income to get back into the game, either for themselves or the teen children that they now had.
2. Pop culture, with shows like Stranger Things giving us old nerds the nostalgia buzz and reminding us of our D&D glory years, and YouTube providing a space for the actual play shows that made the game understandable to new players.
However, I mentioned Pathfinder, 1.0. I do think it made a massive contribution to 5e because it basically preserved D&D in its more traditional form and forced WotC to reassess the design choices they had brought in for 4e. If not for Pathfinder providing stiff competition, WotC might not have course corrected and 5e would not have been as comfortable for the old nerds to come back to. And no OGL, no Pathfinder 1.0.