We are already near it, but I think once a lot of media has been created purely out of AI, especially full length films, that will be the one to wake people up. The government wouldn't know how they can make money out of these. AI replacing actors and voice actors aren't paid, and don't have taxes. It would probably be the company that created the product that will pay taxes based on the revenue. I agree that they might need to create or use something aside from GDP very soon.