The best thing Wizards of the Coast ever did with 3rd and 5th Edition D&D was to create the OGL. Were there a lot of terrible adventures and gaming supplements created under the OGL? Absolutely. Will there be many more terrible adventures and gaming supplements created under the OGL until the end of time? Probably.
By freeing people to work within the existing D20 framework, Wizards of the Coast allowed others to use the D20 mechanics, but create their own worlds / adventures / monsters / etc and sell it. Of course, people have always been able to do this own their own, but by allowing these ideas to be sold to the public, so that imaginative and creative people could profit off of their creations, inspired those people to create more content.
You could argue that a person could always create their own game with their own mechanics…
…but I find that by allowing people to “play in their sandbox”, Wizards of the Coast has effectively extended the life span of their OGL games almost infinitely. After all, not only could the creators of OGL content play in WotC’s sandbox, but they could play in each other’s sandbox as well.
With the rise of the internet and self-publishing, we are ripe for a real renaissance in DIY roleplaying. If you have a great idea, whether an adventure, a monster, a character class, a setting, make it! You do not need permission to create it. Just do it.