My non professional idea would be that you're getting small publishers free riding off the bigger publisher. How many people are buying D&D in order to play amazing third party products?
Just glancing over at my shelves, I have the full complement of Lazy DM books, Roll & Play: The Gamemaster's Fantasy Toolkit, Magical Industrial Revolution, The Dungeon Alphabet, The Monster Alphabet, The Stygian Library, Ptolus: City by the Spire, Ptolus: Secrets from the Delvers Guild, Ptolus: City of Adventure, Monsters of the City, The City Watch, Empire of the Ghouls, Monsters of the Underworld, Monsters of Feyland, Fey Encounters, Latin American Monsters, An Unexpected Wedding, Incantations, Adventures in Oz: Campaign Setting, Adventures in Oz: Beasts & Beings, and the complete run of Goodman Games' Original Adventures Revisited (those could probably exist under the new license, since they're a special agreement with WotC).
The WotC stuff I have out and available: PHB, MM, DMG, Xanathar's, Tasha's, Monsters of the Multiverse, Wild Beyond the Witchlight, Tales from the Yawning Portal, Van Richten's, Strixhaven and Journeys Through the Radiant Citadel.
This doesn't count what I've got in storage (less than I've got out -- I'm pretty minimalist nowadays), in PDF form or via D&D Beyond.
I DM for more than a dozen of folks, many of whom have started DMing as a result of our long-term campaign, generating lots more sales of WotC core books and supplements.
WotC is not losing out on money because of my third-party purchases. They don't have comparable material that I'm choosing not to buy. Likewise, they are not crowded out of the marketplace by the presence of these third party products. If they wanted to create a Jane Austen-inflected adventure, the existence of An Unexpected Wedding wouldn't even rate as a speed bump to them.
The presence of a vibrant third party scene helped bring me back to D&D. The loss of it would not suddenly mean I'd be buying Ravnica or Storm King's Thunder. I would just play with what I've got and do even more homebrew.
The OGL helps everyone
and expands the hobby. A gaming industry without it would be smaller and have a significantly smaller audience, along with a much smaller pool of designers for WotC to draw from.
Tightening up or shutting down open gaming is a stunningly stupid idea.