Ironically, they only did that in the first place because WotC tried to get rid of the OGL. Had 4e been published under the OGL, Paizo would have been happy to just keep making 3rd party content for it. It was the GSL and its “poison pill” that made Paizo go “well, guess we’ll just keep making 3.5e then.”
What’s unreasonable is D&D’s owners spending decades benefiting from having everyone and their mother making content for D&D on their own dime, all the while promising they’d be able to keep doing so forever, only to pull the rug out from under them and say “now start paying us or shut down.” Forgive me if I’m more concerned about what’s fair to the small independent publishers than what’s fair to the billion dollar corporation.