Doesn't ChatGPT probably regurgitate stackoverflow answers? If stackoverflow doesn't exist, ChatGPT probably also gets worse as it doesn't have the data necessary to be able to answer questions.
Besides, Github, Gitlab, Bitbucket, etc. have far more public data to train on, Stackoverflow only really provides the schema one would want for fine tuning to make sure the answers are in the right general question-code-answer-code format.
Once that is trained properly (see: to the understanding level of compression, not just memorization) you only need to update the model on new libraries and languages, which can be trained from repos and docs directly.
Besides, Stackoverflow runs ads and profits directly from users' posts. You don't get to own things just by the virtue of being a platform that lets people host things. Does Github own all the people's code they host? Does a cloud service own the software people put in their containers? Of course not. Stackoverflow themselves state that each answer is the property of the poster, licensed as CC BY-SA. Even Reddit's TOS states that "You retain the rights to your copyrighted content or information that you submit to Reddit".
Now whether there should be a class action lawsuit including everyone who's ever posted on Reddit or Stackoverflow against OpenAI is another story.
They ran adds to cover the servers and development cost. A fork must find how to get money.
It'd only get worse in cases where only a human had access to the knowledge or the means of attaining that knowledge.