Yes 25 years ago if you wanted to buy gaming supplements in the UK certainly you had to go to a specialist game store. The main bookshops might carry a core rulebook, maybe. Not likely. (Edit: Just seen ruin explorers post - that might have been in London, but certainly not in the wider UK - though his timescale is 15 years before mine)
The argument made in the linked post was that D&D (the brand) benefited from microcosms of FLGs stores because it stimulated the market. I don’t believe it works that way in practice. D&D is the gateway, not the other way around. People but the D&D books and then get level up. They don’t buy Level Up if they’re not interested in D&D and then switch to D&D later on.
D&D is only the gateway if enough people play it. To keep it as the gateway, funneling money back to you, you need to ensure the maximum amount of people play your game or use your rules. You accomplish this by getting the game so big that everyone plays it and keeps playing it. It's not just about getting people in, although that is a major part, it is also about keeping people in the bubble. So, person A buys the PHB, and starts playing. Then they figure "I want more out of this game, there are things I feel I can't do in it, I want it to work in a different way". So, in the absence of third party support they go out and they ask "how do I get this in my games" and then they find other games and start playing them. If instead there are third party producers out then they find Level Up, or whatever other third party book the player finds that makes the game work the way they want, they go and buy that and stay in the system. They're likely to spend money on more Hasbro books later on, but even if they don't they're still there as a potential player for new players coming in, when they talk about gaming they talk about D&D and if they introduce people to gaming they introduce them to D&D.
You can never think about specifically in terms of one person and how Hasbro gets the most money out of them, because then you are entirely missing the network effects. It's about the totality, and in that sense WotC has done nothing but profit from the OGL and the D20/D&D bubble.
Here's another example. When I was a kid, I started playing with other games. I was introduced to roleplaying in the mid 1990s via The Fantasy Trip. I then got my own first game, the 3rd edition of the Swedish BRP game Drakar och Demoner (now coming out in its 11th edition as Dragonbane in English from Free League) and started GMing for it. My third game was AD&D 2e, first a starter set and then over time the three core books. I played this a bit, but moved on to other games. I played lots of different gams, many Swedish ones, lots of BRP. I tried free form stuff, indie games, Unisystem stuff. But from the late 1990s to 2013 I bought not a single D&D product. Nothing. Then in 2013 I stumbled upon the Fellowship of the Bling actual play on rpg.net. I read it, and it made me want to play old D&D again. I bought pdfs of Rules Cyclopedia and B/X, I got Labyrinth Lord and then i got Lamentations of the Flame Princess.
By that point I had gotten really into the OSR, getting many of the free and cheap rule sets. I also started buying old TSR adventures and new OSR stuff and eventually even got into buying a bunch of the AD&D 2e books I never had as a kid, like the Complete Handbooks. I ran LotFP games, and by this point I was partway into the D&D bubble again. I did still play and run other games (and the TFT relaunch really got me into TFT again) but OSR stuff was taking up a large part of my time. And then I started getting interested in 5e. I got the corebooks, liked the game, got into a few campaigns, started getting more and more books and when one of my Lamentations campaigns ended with a total party kill, I suggested running 5e for the same crew. So we did. I also had at this point left the staff of Sweden's largest tabletop gaming convention, and after running TFT last year decided to run a D&D 5e game this year. That would have been big because the con hadn't had a single D&D game run at it in at least 15 years, and the one ran then was something called Dungeon of Doom which was a drop in game where you saw how far into a dungeon designed to kill you you could get with a pre-gen, with all dungeon changes permanent (so if a group played before me and killed a mimic, when I played next the mimic would be dead). No serious D&D game had happened for two decades. And now it's going to be at least another year because the planned game was scrapped as soon as the news about the OGL broke, because no way in hell was I or my fellow game runners going to be promoting anything from Hasbro after that.
So, from this story we can see that I would have never gotten back into D&D if not for the OGL. Because without the OGL, no OSR. Without the OSR very likely no Fellowship of the Bling and no old D&D pdfs. Even if those had still existed, I don't think my interest would have been maintained if not for the OSR. And without me playing OSR games which got me partially back in the D&D bubble I hadn't been in since the 90s, I wouldn't have gotten into D&D 5e. So I would not have bought all the old pdfs I bought and I would not have bought all the 5e books I bought if not for the OGL allowing the OSR to flower. This is the type of roundabout and indirect effects a large bubble can have, and the type of journey that just looking at some kind of "assumed customer" won't tell you.
The best argument I’ve heard is that experienced DMs interested in 3pp bring on new players who then go out an buy D&D players handbooks. I’m skeptical as I don’t see any evidence that these groups even make up a significant
Portion of the player base.
That's also a group, that if the third party material was not available would have left D&D. And again, everyone counts. If there are fewer people in the D&D/D20 bubble, that means some new gamers will be brought in through other games. That means the bubble is not working well as it isn't holding on to as many people as it could and it isn't getting all of those new gamers either. Also those people buying third party stuff are more likely than not to be GMs, and those are the biggest hurdle in getting someone playing. To join a group there needs to be a GM.
