This is very debatable. Firstly Fate was an OGL use of Fudge, and Even Fudge would be considered a very small outlier small game.
Without providing factual numbers your assumption that the OGL helped Fates Popularity is pure speculation.
Even with the Kickstarter, 10,000 backers is Not a very large share of the market. It is also more likely that the Swell of the Kickstater Fad had more to do with it's Kickstarter success than any options of OGL publishing.
Dungeon World is a game with an even smaller following than Fate. It's Kickstarter didn't even have a 10th the backers that Fate had. So again, trying to make claims that an OGL Helped these Niche Market games within a Niche market is a huge stretch.
You would need some serious Concrete evidence to show that the OGL (reardless of people actually using the OGL to publish related products ) had an Impact on their popularity. I think you will be hard pressed to find such evidence.
As I've said, I'm no expert.
I think it's quite evident it had a huge impact on the d20 system popularity, but as Nicholas said, WotC is on a completely different level.
Yes, Fate and DW are a niche within a niche. Rolemaster is the same (actually, it's probably even more a niche than Fate, at the moment).
Hell, everything that isn't d&d is a niche within a niche! The RPG market isn't that big.
Do I have proof on the OGL impact on Fate/DW popularity? Of course not! I'm just curious, that's why I asked.
and supplement released is basically free advertising for the main game.
And this statement right here is the equivalent of telling an Artist or Musician that they should provide work in exchange for exposure.
Hmmm, no, as I see it, it's not really the same.
A more fitting analogy would be the Musician letting a tribute band perform and sell their own CDs (do they even sell CDs anymore?
), provided they have a sign that says "Official XXX Tribute Band" when they play or put a sticker with the same phrase on their CDs.