I've never spent much time with formal game theory, so I have the feeling that most of this went over my head, but this stood out:

when the cost is zero to post, comment, engage or any activity, the rational strategy is to maximize extraction regardless the quality of the post.

Maybe extraction is kinda always the name of the game in social media whether people want it to be or not. And all the various distortions we see (advertising, moderation, algorithms, etc.) are attempts to place bounds on extraction.

But in the case of SN, requiring users to pay before extracting puts controls on it that are more directly tied to the problem itself.