I don't think its as insider as you are suggesting (which I guess is the point of contention). Granted a newbie won't know, but anyone who has played for a while is going to pick up on some of these things. Its just the nature of people to learn things relative to that which they find interesting or enjoyable. Do I think its 50% of the DnD Audience? Maybe not. Do I think its greater than 1%? Most definitely.
The 1%/0.1% number is people who really care about there being or not being an OGL. And who might care enough to actually think about writing under the licence.
It really might be that low.
Realizing a product not made by WotC is a big step. Not everyone who plays will even be able to tell you the publisher of D&D. They know the game, they enjoy the game, they might have even bought a PHB, but knowing who the publisher is, is a big step. Again, the majority of gamers who identify as "D&D players" might not even own a PHB, and rely on the DM (or dedicated player) for their books.
So mentioning a different publisher name will not mean much (even the term "3rd Party" might not mean much).
We also live in a video game era, so things will be compared with that. Most video game consoles have regular games not made by the same publisher as the console. That's just something that happens and even regular video game players might only have a passing interest in how a 3PP video game company gets approval to made a compatible release.
So even IF they're involved enough to know who publishes D&D and know that Company X isn't WotC, they might not know about the OGL or care about the details.
For those people who are interested in the nuts and bolts of D&D enough to learn about Open Gaming and know of the licence, most won't be interested in writing. They might be happy to buy 3PP books, but details of the licence not a huge concern. Or their attitude is that, since it's not official, they don't want it in their game. And, again, the OGL is not a huge concern.
And those few that are interested that are might just be fans willing to post stuff on their website. And a document full of legalese is overkill in that regard. Really digging into the nuances of the licence or having strong feelings are not needed to throw up a race or campaign setting on blogger.
Even people who are willing to use 3PP need not be concerned with the details of the licence. If they have the product, how it was made and the details don't matter. Whether it was the 3e OGL or a new OGL or something else doesn't *really* matter to them.
And, of course, of those people who do know about the OGL and the details, there are those who don't want it again. The detractors who say that open gaming was a mistake, that it hurt D&D, that WotC shouldn't make a new licence.
Gamers who know who the publishers are, know how the 3rd party licences work, know how the OGL works, and want 3rd party material are a really small subset of the gaming audience.
I think they're an important part, and healthy to the industry, and the OGL is something I personally want. But it's not a big part of the gaming populace.
Heck, even looking back at my own experiences, I know of open gaming and 3PP very quickly. But I didn't *really* know details of the OGL until the GSL came out and I started comparing the two. But I'm not neurotypical and obsess over things, so I doubt very much I'm representative of the general gaming public, so my focus on the OGL is likely abnormal.