@chris I suppose one thing to consider is if a federated pool of knowledge is CC-BY-SA, then we only need a court ruling that OpenAI violates CC-BY-SA and the federated pool becomes AI-safe. Whereas SO can, (or already has) change the TOS so they own rights to relicense all content.
…but of course, CC-BY-SA is also incredibly inconvenient for a SO clone because everyone will generally want to copypaste sample code!