There are several moving aspects of the open gaming license over these last 17 years of open gaming. These are un-cited opinions based on observations and intuition.
First, it opened the 3rd edition rules of Dungeons and Dragons to legal and safe tinkering, derivation, and expansion. An industry sprung up to create supplements for 3E. It also empowered people to create gaming content that was shareable; In a legal manner as well as in proximity or alignment to existing rules systems.
Second, it provided the framework to legally rebuild older variants of Dungeons and Dragons. First in the form of OSRIC, then the ensuing explosion of clones, modern simulacra (in no particular order), and adjacent games:
- Labyrinth Lord
- Basic Fantasy
- Sword & Wizardry
- Lamentations of the Flame Princess
- Mazes & Perils
- Castles & Crusades
- Delving Deeper
- Adventurer Conquerer King
- Dungeon Crawl Classics
- The Hero’s Journey
- The Black Hack
- Microlite ‘74
- Beyond the Wall
- And I’m sure more
Third, it was through the OGL that Pathfinder came into existence. Pathfinder is the spiritual and mechanical descendant of the most popular modern rules of D&D at the time. The rules and community had of the very popular rules had a safe harbour as D&D 4E experimented with a new paradigm and a non-OGL license.
Fourth, I look to WotC’s releasing 5E D&D under the OGL as a sign that the OGL is all-around better for them than not. Wizards of the Coast took this a step further and through the DM’s Guild have released their flag-ship RPG campaign world, the Forgotten Realms, for experimentation.
In the middle of these four points we see great developments. Old D&D adventures, long out of print, available again in PDF. You can use a simulacrum or the original rules to play these adventures.
We are also seeing old adventures (B1 - In Search of the Unknown and B2 - Keep on the Borderlands) being re-released by Goodman Games under both 1E and 5E rules in a single volume.
The open game license appears to have created a framework for iteration and experimentation all while ensuring that gaming and the DIY nature of gaming remains viable.