I'm not as sure about the commercial implications for WotC. I'm not denying that there are some sales made by them that are driven/supported by 3PP materials, but I have doubts about how big this is as a proportion of the overall recent growth of D&D.
Oh? And where is this magical manpower that Wizards conjured up that singerly handily expanded the D&D to its largest audience ever?
I understand what you are trying to say here and I apologize for the sarcastic tone. But figuring out how D&D expanded to its largest audience ever is about the logistics of how people become aware of D&D in the first place. And the answer is pretty obvious when you consider the logistics.
Historically Wizard's staff devoted to D&D has been relatively small. Much of their creative and logistical support was outsourced to freelancers and licensed outlets. But those outlets and freelancers are dwarfed by the number of people actively involved in publishing RPG whether it is 3PP or not. The 2010s saw a huge expansion of creative output across the board enabled by the low barriers to publication created by the internet and digital technology. What is relevant here is not the sales or dollar volume here but the social network effect of all these creatives.
Whatever the ratio is it magnified that most of these folks are referee types which mean they are surrounded by a far larger group of folks just here to play.
But then growing throughout the 2010s is what I would call effective third party promotion. Sure we had blogs, articles, zines, and forums, but the rise of Youtube and more importantly the examples of the effective use of Youtube like the one set by the Critical Role crew meant that now instead mostly referee types being plugged into the wider hobby, we see more the wider player base becoming engage.
The next piece of the puzzle is the goodwill generated by the runup and initial release of D&D 5e. While it didn't have a open content SRD until later the "era of good feeling" meant that for several years D&D 5e became everybody's 2nd favorite RPG. For this Wizards at the time deserve the credit for being the primary movers behind this.
But this set the stage for a multiplier effect on 5e when effective third party promotion started to come into play. As a well-liked system, the decentralized network of promotors started talking up 5e. Then the OGL was released along with popular programs like the DM's Guild, D&D Beyond, and licensed content for VTTs.
The stage was set for a larger consumer market to become aware of the potential of tabletop roleplaying and D&D 5e. When lockdowns of the pandemic came into the picture, many turned to the internet for their primary form of entertainment and social interaction. The odds of an unconnected individual stumbling on something RPG or D&D related was at a high point. This ignited sales of 5e and propelled D&D and the hobby to its high point.
This is the nuts and bolts of how the current market situation with D&D came to be. Wizards is responsible only for the goodwill in the 5e runup, and the then-current licensing situation. The rest was driven by third parties.
If Hasbro wants to "win back their IP rights" is this really directed at Paizo's present position? Or any other 3PP? Or at controlling further growth that they envisage flowing from the anticipated success of the movie? To me, it makes more sense as a future-oriented concern than vindictively trying to destroy the present ecosystem.
This is a case of bad generalship plain and simple. The executive at Wizards are fighting the last war and failed to consider how the battlefield works today. If they were good generals then they would be aware that by restricting people's use of the original content they created is a lines that when crossed will ignite a firestorm of criticism and a collapse of the value of their goodwill.
